university of haifamath.haifa.ac.il/hinich/web/mypapers/ovc.pdfn jv e hq g .n ftw ik\;acb...

38

Upload: others

Post on 08-Nov-2020

0 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

������������ ������������������������������ �!��"�#�$�&%'�$�(�)����'�* ��+�,��-.���(�0/1�$��2�4365 �+�$��879�':;�'����<��

=?>

@BADCFEHGDI1GDJLKMGONPGOQ6RS,TVUFWYX[Z[\]T_^`Z�acbHdeWYZgf�TV\hWYZgikjVlm�^�ion`TVXglpioZ > acbPq,W�ikbDWq,W�ikbDWsrtvucw`xzy�{|lpXgW�T_}

W�^�~

��N���RP��N��������_��HRS,TVUFWYX[Z[\]T_^`Z�acbHdeWYZgf�TV\hWYZgikjVl

� f�T���T_io�_\hW�^�^B{|^�lpZ[ioZg�ZgT�acb$��j_ioT_^�j_T�)T_f�avn`a�Z��6�tvwcwy�{|lpX�W�T_}

Page 2: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

��� � N���Jv��E��HQ���G �.N

������� � f�T�W�ik\;acb�Zgf�iol$UFWYU�T X ikl�Zga U�Xgavn`T�W�Z�� T_^`Z > > T WYX'ac}k~]jVac^|T_jVZg�XgT��� `ryu�� u�Fac^]io^�j_}k��lpikac^�lacb�a�X = i Z�W�}&nYWYXgikTVZ > j_}oacl[�XgT_l�� � f�TB~�T_l[jVXgi U�Zgikac^ acb)ik^�j_}o��l[ikac^�l acb�a�X = ioZgW�}&n�WYXgioTVZ > j_}kacl[�X[T_l]iklsWn`TVX > ~�i�� j_��}oZ U�Xga = }kT_\ W�^�~9TVn`T_^8ik^ Z > U�T�� W�^ T��ZgTV^�l[ion`T�W�^ W�} > l[iol�� d1tcy der� f W�lM^�a�Z > TVZU�Xgavnio~�T_~ W,jVac\ U }kTVZ[T&l[ac}k�Z[ikac^��4l[TVT+��� ����� � �X�l[ac}o�Zgikac^ iol ik^ ZgTVXg\ lHacb j_TVX[Z�W�io^���W�l > TVZ ��^�!z^�a"�)^#�$ T_ac\]T Z[Xgikj'~ WYZ�W�W�^ W�}oa $ ac��lHZga&%'T Xg\hW�\]a?~���}kT \���}oZgi U }kioj_ioZgioT_l�� {|Z'� W�l�\ a�ZgionYWYZgT_~ =�> Zgf�T W�^ W�}ka $ >= T Z(�&T_T_^ $ T_ac\]TVZpX > W�^�~#XgTVU�XgTVl[T_^`Z�WYZgikac^#Z[f�T_a�X > W�l T��U�X[T_l[l[T_~ =�> Z[f�T l[a)�j W�}k}oT_~�a�X = ioZ \]TVZgf�a?~l[ac\]T Zgik\]TVl�X[T_b TVX[XgTV~�Z[a�W�l+*?� W�^�Z[ik� WYZgioac^,�2� f�T_^ ac^�T$UFW�lpl[T_lHZga Zgf�T'l[TV\]ikl[io\ U }kT�j W�lpT'Zgf�ikl2\]TVZgf�a?~T_^�j_ac��^`ZgTVX[l'W�^���\ = TVX�acbF~�i��]jV��}oZgioT_l = T_j_W���l[T+j_a`W�~|acik^`Z�a�X = ioZgl�W�^�~���^�i Z�WYX > XgT U�XgT_l[T_^`Z�WYZgioac^�l-�4a�XU�Xgio\]ioZgi n`T io~�T W�}kl�� ~�aL^�a�Z.*?��ioZgT \]WYZgj�f �U/����T f W n`T l[� $�$ T_l(ZgT_~0�1 �xzyHt�� x2�HWhj_acf�TVXgT_^`Z.�+W > ac�Zacb'Zgf�iol�~�i3�]j_��} Z > ��4,W�\]T_} > j_TVX[ZgW�ik^#WYZpZ[Xgi = �Z[T_l acb'Z[f�T $ T_ac\]T Z[Xgikj W�^�~#acb$Zgf�TsXgT U�XgT_l[T_^`Z�WYZgioac^ W�}U ikj Zg�XgT_lsj W�^ = T�j_ac\ UFWYX[T_~ W�^�~�Z[f�T_^�Zgf�T_i X ~�T Z�W�ik}oT_~�~�T_lpjVXgioU�Z[ikac^ iol Zga = T $ ion`T_^ =?> T���W�jVZg} >Zgf�T lgW�\ T�W�} $ a�Xgi Zgf�\]l io^eT W�j�f�j W�lpT = �Z =?> ��l[io^ $ l[}ki $ f�Zg} > ~�i35�TVXgTV^�Z�~ WYZ�W���{|^MZgf�ikl�bDW�lpf�ikac^eZgf�Tj�f WYX�W�jVZgT XgiklpZ[ikj U.ac} > ^�ac\ i W�}kl,acb a�X = i Z�W�}�n�WYXgioTVZgioT_l6� T XgTh~�T87 ^�T_~9�1 `ry$��TVjVZ:�<;2�&W�^�~�j_ac^|TVjVZg�XgT_~�1 `ryFu��>=��Z[a = T T��zU�XgT_l[lgW = }oT io^ ZgTVXg\ l acb $ T_ac\]TVZpXgikj�~ WYZ�W?|��lpZ�W�l.@�ac}k~�ioT�XgW�^�!�U.ac} > ^�ac\]i W�}ol+WYXgT$ ion`TV^#ik^ ZgTVXg\]l�acbA%$TVXg\hWL\]az~���}oTs\���} ZgioU }oikj_i ZgikT_l8�L��TVn`T X�W�}�U�X[azacb l6�&TVXgT]}kWYZgTVX $ ion`T_^B� � yC ?xzyD%&�E�� f�ikl,XgT_l[��} Z W�}ol[aB~�TVl[jVXgi = T_~#Zgf�T $ T_ac\]T Z[Xgikj�j_TV}k}klVy�Z[f WYZ iol,Z[f�TF7 = XgTVl acb$Zgf�Th�`ZgT_io^ = TVX $ \hWYU�y�ik^ZgTVX[\]l&acb�Zgf�T,W = a�n`T $ TVac\]TVZ[Xgioj ~ WYZ�W W�^�~ $ W n`T�WG�+W > Zga jV} W�l[l[iob > ^�io}oU�a�Z[T_^�Z+a�X = ioZgl&lpa Zgf WYZ&Zgf�T_i XXgT_}kWYZgikac^#Z[aeU�X[ik\]i Zgion`T]io~�T W�}kl = T_j_ac\]TVl Z[X�W�^�lpUFWYXgTV^�Z:�H� �X U�X[T_l[T_^`Z�XgT_l[��} Zgl�~�T_TVU�TV^ W�^�~1XgT87 ^�TZgf�iol�XgT_}kWYZgikac^�lpf�ioU1~�T_l[j Xgi = io^ $ ik^#UFWYX[Zgioj_��} WYX�Zgf�T $ T_ac\ TVZ[Xgiojsj_ac^�TVlI� f�ioj f�~�T ZgTVXg\]io^�T a�X = i Z W�^�~a�X = i Z�W�} nYWYXgioTVZ > j_}oacl[�XgT_l��){|Z[l�ik^`ZgTVXgT_l(Z,\hW > � T_}o} $ a = T > ac^�~el[T_\]iol[ik\ U }oTGJ ikT W�} $ T = X�W�l,W�l Z[f�TVXgTikl W�^ W�^ W�}ka $ ��TBacb�Zgf�T �`Z[T_ik^ = TVX $ n�WYXgioTVZ > io^�Zgf�T�Zgf�T_a�X > acb�UFWYZgf W�} $ T = X�W�l_y l[TVTK� �)T�y>LM4?� b a�XT���W�\ U }kTN�

���PO#� {|^ \ a�XgT]~�TVZ�W�io}&}kT ZRQ = TBWMj_ac\sU }kT��#l[T_\]iol[ik\ U }oTSJ ikT�W�} $ T = XgW W�^�~T7���WVU WYXpZ�W�^1l[� = )W�} $ T = X�WXWY��{(^�tvu?�c�sS,��Z a�� S&� lpf�a"� TV~ Zgf WYZ,TVn`TVX > U�Xgik\]i Zgion`T ik~�T W�}Hacb�Zgf�T�T_^`n`T_}ka�U io^ $ W�} $ T = X�W[\��Q���ikl Zgf�TLW�^�^�iof�ik}kWYZga�X acb W l[io\ U }kTsf�i $ f�T_lpZR�&T_i $ f`Z�\]az~���}oT�yPZgf���l $ ionzik^ $ W lp�X]|T_jVZgi n`TL\hWYUb Xgac\ WN^ Zga`_bacEd [\��Qe����f = W�l[ikjg*z��T_l(Zgikac^V�+W�l�Zga ~�TVZ[TVXg\]io^�T Zgf�Tg7 = X[T_l acb'Zgf�T]S,��Z aM\hWYU/�

hjikh

Page 3: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

m�l[io^ $ Z[f�T�j_T_^`Z[XgT�acb [R��Q���W�^�~LZgf�T�q,WYXgiklpf ) U'f W�^�~X�W iol[ac\]a�X[U f�iol[\ ac^�T�iol&XgT_~���j_T_~�Zga�lpZg��~ > ik^ $7 = XgT_l acb \hWYU l b Xgac\�Zgf�T �#T > } $ Xgac�U�� Zga`_bacEd [\��Q�� �]{|Z6�+W�l ^ WYZ[�X�W�}�Z[a = T_}oikTVn`T Zgf�ikl�Zgaf W n`TLW j_ac\ = ik^ WYZga�XgikW�}$lpac}k�Zgioac^,����f�a�X[Zg} > W�b ZgTVX S���Z a�� lI� a�X !V�&T]^�a�ZgT_~1Zgf WYZI%$TVXg\hWB\]az~���}oT\���}oZgi U }kioj_ioZgioT_l�l[f�ac��}o~�U }kW > W !`T > X[ac}kTN� � f�iol�j_W�^ = T T �U�XgT_lpl[T_~ =�> W�l[l[i $ ^�ik^ $ Zga�T W�j�f�����W�^#WYU�U�X[a�U�Xgi WYZgT\%$TVXg\hWL\]az~���}oT�y�l[aBZ[f WYZ�Zgf�T l[io\ U }kT�f�i $ f�TVlpZG� TVi $ f`Z \]a?~���}kT_l $ W n`T W�l[T_j_ac^�~= W�l[iolR�4^�a�ZgT_~�#���6�� ������ ik^ �� �tcy�x � � �PW�^�~MlpT_T ��� ; �&acb���� �&��T ^�a�ZgTV~ Zgf WYZ Zgf�iol j�f W�^ $ T acb= W�l[iol $ ion`T_l,WSZFW $ acb�}kT_b Z,io~�T W�}kl�acb���� W�^�~�lp� $�$ T_lpZgT_~eZgf WYZ�Z[f�T io^�j_}k��lpikac^MXgT_}kWYZgikac^�l = TVZ�� T_T_^Zgf�T_\ l[f�ac��}k~LT���W�jVZg} > ~�T_l[jVX[i = T�Zgf�T�io^�j_}k��lpikac^hXgT_}kWYZgikac^�l&acb�U�Xgio\]ioZ[ion`T io~�T W�}kl?�4l[T_T����>=e� � � f�iol �+W�lUFWYX[Zg} > TVlpZ�W = }kiol[f�T_~ �1 �tcy.x �>= �4ioi � �2W�^�~ Zgf�T�U�Xgazacb j_ac\sU }kTVZgTV~ =?> %'a $ W�^K� %&�HW�}o\]aclpZ)ik\]\ T_~�i WYZgTV} >W�b ZgTVX �+WYXg~�l��8{�Z �+W�lLWV!`T > \]a�Zgi n�WYZ[ikac^ b a�XLZgf�T � a�X ! acb���W��_f�~ W�^ W�^�~BJ ��l[�VZ[i $ ���bJ$t��?�)f�aj_ac^|T_j Zg�XgT_~�W�U �X[T_} > j_ac\ = ik^ WYZga�X[i W�}�W�} $ a�X[ioZgf�\0ZgaB~�TVZgTVX[\]ik^�T Zgf�T� ���6��y/�)f�acl[TsnYW�}kio~�ioZ > �+W�lT_lpZgW = }kiklpf�T_~ l[f�a�X[Z[} > W�b ZgTVX � WYXg~�l =�> l[TVn`TVX�W�}2W��Zgf�a�XglR�1L L�y>L��.�E�

�����,� {(~�TV^�Zgiob > Q ^ �)ioZgf Q]ZgfXgac� $ f Z[f�T���ik}k}oik^ $ b a�Xg\ W�^�~ }kTVZ Q��! #" W$"% '& = T WsZpXgi W�^ $ ��}kWYX~�T_j_ac\sU�acl[i Zgikac^,�Rf j_a`W�~|acik^`Z a�X = ioZ)( ikl�lgW�ik~ Zga = T ^�io}oU�a�Z[T_^�Z�ikb*(,+- eikl�^�ac^ ) TV\ U�Z > �Ff�b Z[TVX�`UFW�}oZ[T_^�lpZgT_io^.(/+# ]ikl T8*z��ik~�io\]T_^�l[ioac^ W�} � �Ft���W�^�~Bac^�T�j_W�}k}kl i Zgl ioXpXgT_~���j_i = }kT,j_ac\ U.ac^�T_^�Z[l a�X = ioZ�W�}nYWYXgikTVZ[ikT_l�� � ^�T,\hW > l[f�a � �� �ryF� � x2�FZgf WYZ W�^La�X = ioZ�W�} nYWYXgikTVZ > iklMJ2W $ X�W�^ $ i W�^S�)ioZgfhXgTVlpU�T_j Z Zga Zgf�T��ioXgio}k}kavne)0��acl(Z�W�^�Z l > \sU }kT_jVZ[ikj b a�X[\ ac^1(0W�^�~�f�T_^�j_T�W ^ WYZg�X�W�}$a = |T_jVZ Zga<*z� W�^`Zgik�_T�BS,T_lpU i ZgTT WYXg}oikTVX2a�U�Zgio\]ikl[\9^�a�ZPW�}k}`a�X = i Z�W�}�nYWYXgikTVZ[ikT_l j W�^ = T *?� W�^�Zgio�_T_~ $ i nio^ $ TVn`TV^�Zg� W�}o} > W U�Xgio\]ioZ[ion`T ik~�T_W�}�1 `�yFt�� r��2 = �Z&Z[a ~�TVZgTVX[\]ik^�T Zgf�acl[T �)f�ikj�f]j W�^ = T�ikl'W6!`T > a�U�T_^sU�Xga = }kT_\ ac^��)f�ikj�fhl[ac\]T XgT_j_T_^`ZU�Xga $ XgTVl[l,f W�l = T_T_^eXgTVU.a�X[ZgT_~ ��L,yC `d1y43 �]��q�TVX[TI� T |��lpZ,nioT8�"W�^ea�X = i Z�W�}2nYWYXgioTVZ > W�l�Zgf�T ^ WYZg�XgW�}$ T_ac\]T Z[Xgikj�W�^ W�}ka $ ��T�acb W U�Xgik\]i Zgion`T)ik~�T W�}�� {(^hZgf�iol&Zgf�T �`ZgT_io^ = TVX $ \hWYUhXgTVU }kW�j_T_l Zgf�T�S,��Z a \hWYUW�^�~MZgf�T j_ac^�a�Xg\]W�}kl acb L&Xg��f WYZ�jVT_}k}ol�ik^ Zgf�T j_a�ZgW�^ $ T_^�Z = ��^�~�}oT�ac^ Zgf�TIZFW $ n�WYX[ikTVZ > WYX[T nikT�� T_~W�l�Zgf�T $ T_ac\ TVZ[Xgioj W�^ W�}ka $ ��T_l�acb�l[io\ U }kT f�i $ f�T_lpZ.� TVi $ f`Z,\]az~���}oT_l��65 io^ W�}k} > Zgf�T = W�l[T�j�f W�^ $ T�ik^��� ^�T_jVT_l[lgWYX > Zga $ i n`T]W�^�W�^ W�}oa $ ac��l�~�TVl[jVXgi U�Zgikac^�acb&io^�j_}k��lpikac^#acb&a�X = ioZ�W�} nYWYXgikTVZ > j_}oacl[�XgT_l iolU�Xgavnio~�T_~MZgfXgac� $ fMZgf�T)� W�j Zgikac^Mac^ Zgf�T\L a�XgTV}3)�dMa?a�XgT f�ac\]ac}oa $ > acbHZ[f�T87pj_ac^�a�Xg\hW�}�n�WYX[ikTVZ >:9 �q�TVXgT�Zgf�T b ��^�~ W�\ T_^�ZgW�} j_}kW�l[l[T_lsacb�Zgf�T j_ac^�a�X[\hW�}kl b a�Xg\ Zgf�TBXgT8*z��ioX[T_~ = W�l[iklg�\���6�; <�=���

acb>�#� ^�TVT_~�T_~ Z[a ~�TVl[jVXgi = TLZgf�T $ T_ac\]TVZ[X[ikjhW�^ W�}oa $ ��T_l acb)jVac^�T_l W�^�~ j_TV}k}kl8� � f WYZ Zgf�ikl iol Zgf�T

h@?kh

Page 4: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

WYU�U�Xga�U�XgikWYZgT,ik~�T_^`Zgi37 j WYZgikac^�ikl+\hW�~�T�UFWYXpZgikj_��}kWYXg} > TVnio~�T_^�Z =�> Zgf�T,ik^`ZgT $ X�W�}�b a�X[\ ��}kW acb �� e;zy�� � � �E�

����� � � ^�j_T�ac^�Thf W�l���^�~�TVXglpZga?az~�f�a"� Zgae\hW!`T Zgf�ThjVa�X[XgT_jVZ j_ac^�l(Z[Xg��jVZgioac^�l = W�l[T_~�ac^1Z[f�T_l[TW�^ W�}ka $ ioT_l]Zgf�TeU�Xgazacb�acb�ac�XB\]W�ik^ Zgf�T_a�XgTV\ x � x�~�T_l[jVXgi = ik^ $ a�X = ioZ�W�})n�WYXgioTVZ > j_}oacl[�XgT_l�b ac}k}oa"�)lXgT_}kWYZgion`TV} > T W�lpik} > b Xgac\ !z^�a"�)^ XgTVl[��}oZgl W�^�~ ZgTVj f�^�i�*z��T_l8� f = W�l[ikjBik^ $ XgT_~�ioT_^`Z iol Zgf WYZ Zgf�TLZga�UL a�XgT_} )�dMaza�XgT f�ac\]ac}ka $ > ac^hZgf�T��`ZgTVik^ = TVX $ nYWYXgikT Z > f W�l+W�^LW�} $ T = XgW lpZg��jVZg�X[T�\]W!ik^ $ ioZ iol[ac\]a�X )U f�ikj�Zga)��� � � f�iklM�+W�l ^�a�ZgT_~�ik^hUFWYX[Z[ikj_��}kWYX =?> @�ik^�� = �X $ �C@ �E�?q�TVXgT?� T,b �X[Zgf�T X)^�a�Z[T,Z[f WYZ Zgf�TZga�U L a�XgTV}3)�dMa?a�XgT f�ac\]ac}oa $ > ac^ Zgf�T j_ac^�a�Xg\]W�}�nYWYXgioTVZ > iol�W]\]a?~���}kT b a�X)Zgf�ikl�W�} $ T = X�W W�^�~ Zgf WYZ= a�Z[f�WYXgTh^ WYZ[�X�W�}k} > ikl[ac\ a�X[U f�ikjLW�l�� \]a?~���}kT_l�� � f�T�X[T_} WYZgioac^�l[f�ioU = TVZ�� T_T_^�Z[f�T_\ ikl�W�^ W�}oa)$ ac��l)Z[asZgf�TGL+T Xg^�lpZgT_io^ ) @�T_}kbDW�^�~eT�*?��ionYW�}kT_^�j_T��1L @?� = TVZ�� T_T_^ Zgf�T�q,WYXgiklpf ) U'f W�^�~X�W]W�^�~ f�i $ f�T_lpZ� T_i $ f�Z&j WYZgT $ a�XgioT_l�� � f�ikl&}oT W�~�l'Zga W $ T_ac\]TVZpXgikj)W�^ W�}ka $ ��T)acb�Zgf�T��'^X[i $ f`Z&b ��^�jVZga�XM�)f�ikj�f]U }kW > l&Wl[i $ ^�i�7 j W�^`Z+Xgac}kTS�1 ;zy x � ��y x �>=��ik^�~�T_l[jVXgi = ik^ $ U�Xgik\]i Zgion`T ik~�T W�}�io^�j_}k��lpikac^�l���{|^�~�T_T_~ Zgf�iol+ikl f�a �9� Ta = Z�W�io^BW Xgi $ f�Z W�jVZ[ikac^�ac^LZ[f�T�jVac^�a�Xg\hW�}.nYWYXgikT Z > X�WYZgf�TVX+Zgf W�^LZgf�T,}kT_b Z W�j Zgikac^�� f�ioj�fBiol io^�f�TVXgT_^`Zik^ q�a�Z[Z�W:� l � a�X !T� q?�]� fk!`T > a = lpTVX[nYWYZgikac^ io^ ac�X?� a�X ! iol)Zgf WYZ a�X = ioZ�W�}�nYWYXgikTVZ > j_}oacl[�XgT_l�j W�^ = TT��zU�XgT_l[l[TV~ ik^�ZgTVXg\]l acb2l[TVZ Zgf�T_a�XgT Zgikj,j_ac^`n`ac}k�Zgioac^ acb ioX[X[T_~���j_i = }oT j_ac\sU�ac^�T_^`Zgl acb Zgf�T �`ZgT_io^ = TVX $nYWYXgikTVZ > �)ioZgf�j_ac^�a�Xg\hW�}ol�� {�Z�Zgf�TV^ XgT_\hW�io^�l�Z[aBl[f�a �;Zgf WYZ�\ ��} ZgioU }oikj WYZ[ikac^eik^�f�ac\]ac}ka $ > 7plpT_T_l 9W�}k} Zgf�T ioX[XgTV~���j_i = }oT j_ac\ U.ac^�T_^�Z[l]acb�l[TVZ Zgf�T_a�XgTVZ[ikj jVac^�n`ac}o�Zgikac^,� � f�ikl]iol]W ~�TV}kikj_WYZgT U�acio^�Z]W�lik^`ZgTVX[l[T_jVZgioac^�l�bDW�ik} Zga = T U�X[a�U�TVX:� q�a"� T n`TVX�T_^�ac� $ f ik^`ZgTVXglpT_jVZgioac^�l,WYXgT U�Xga�U.TVX�W�^�~eb �X[Zgf�TVXg\ a�XgTWhXgT�*z��i XgT_~�T�*z��io~�ik\]T_^�lpikac^ W�}ki Z > XgT_lp��}oZgl�� � f�ikl�XgTVl[��}oZ�ikl�X�WYZgf�T X�b a�XpZg��ioZgac��l�ik^eZgf WYZ�ac^�T j W�^�^�a�Zf�a�U�T)Zga a = Z�W�io^]T��zU }kikjVioZ$b a�Xg\���} W�TN�H{�Z'X[T_l[��}oZ[l&b Xgac\ ��Xg��}o}�� l$Zgf�T_a�X[T_\ io^hW \hW�^�^�TVX&W�^ W�}ka $ ac��l�Zga�`UFW�}oZ[T_^�lpZgT_io^ � l�U�Xga?acb � �Ft��2acb Zgf�T T�*?��ik~�ik\ T_^�l[ikac^ W�}oioZ > acb ( + �>5FX[ac\ Zgf�T_lpT Zgf�ik^ $ l�jVac\ = io^�T_~ac�X U�Xga?acb XgTVl[��}oZgl8����T�XgTV\hWYX ! Z[f WYZ ik^�ac�XM� a�X !S�&T�lpf W�}k}�ac^�} > XgT_} > ac^`@�ik^�� = �X $ � l+b a�Xg\hW�}oikl[\�C@ � W�l U�X[T_l[T_^`ZgT_~�ik^ � U @ y���T_jVZ��Hr����N�]� � f�ikl \]T W�^�l Zgf WYZI�\���6��iol ~�TVZgTVX[\]ik^�T_~ =�> Z[f�T]Z[X�W�^�l[i )Zgioac^ \hWYZ[X[i3��~�T87 ^�T_~Mio^K�CU-@ yFr���� �ktvr�E� � f�ikl Wvn`acio~�l f W nik^ $ Zga \]W!`T W�^ > j_ac\ UFWYXgiol[ac^ �)ioZgfBZgf�TWYU�U�Xga`W�j�f�T_l�acb � W��Vf�~ W�^ ) J ��l[�VZgi $ � �6J+;2� a�X �)acl[l[\hW�^�^T� �-�E� q�a"�&TVn`TVX)l[T_T�Z[f�T�XgT_\]WYX !Lik^ x � r��

O#� ��h���G ����� � ��RP�����v�zGON����J��#@�C�J�G �?� �

O#����� J2T Z�� = T,W jVac^�^�T_jVZgT_~�y�lpik\ U } > ) jVac^�^�T_jVZgT_~hjVac\ U }kT�� l[T_\ ikl[io\ U }kT W�} $ T = X�W�ioj $ Xgac�U/��5 i3�]W

h�� h

Page 5: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

L a�XgT_}�l[� = $ Xgac�U��;acb �\���)T_j W�}o}.Zgf WYZ����&���������;W�^�~LZ[f WYZ Zgf�T�l[TVZ��acb W�}o}#L a�XgT_}�l[� = $ Xgac�U lacb ��ikl]W�l[ik^ $ }oT ��a�X = ioZ]��^�~�T XLj_ac^|� $ WYZgikac^,� � f?��l� ik~�TV^�Zgi37 T_l��)ioZgf ��� � � {|^ Zgf�iklsioZ]iolj_ac^`n`T_^�ikTV^�Z�Z[a }kT Z���� ~�T_^�a�Z[T T_ioZ[f�TVX Zgf�T,Xgi $ f�Z�� j_acl[TVZ ~�T87 ^�T_~ =�> � � �;a�X Zgf�T�� j_ac^|� $ WYZgT����� &��vy � f�ioj f � T \hW > l[ac\ TVZgik\ T_l W�}kl[aF�)Xgi ZgT�W�l�����O#�PO#� ��TVZ Q���� c�� �k�)ioZ[fLZpXgi W�^ $ ��}kWYX+~�T_j_ac\ U.acl[ioZgioac^ Q�� &�" W "� .y��)f�TVXgT\W�"8 ��� c������J TVZ�� &��� ��� = T�Zgf�T j_a�XpXgT_lpU.ac^�~�ik^ $ j_ac^�^�T_jVZ[T_~el[� = $ Xgac�U l�acb �\�MJ2TVZ"! � X[T_lpU/�!$# �&~�T_^�a�ZgTZgf�T�l[TVZ)acbPXgaza�ZG� XgTVlpU/�$U.acl[ioZgi n`T�y�^�T $ WYZgi n`T�Xga?a�Zgl��$~�T87 ^�T_~ =�> Zgf�ikl)~�T_j_ac\ U.acl[ioZgioac^ W�^�~�%'&(!$)Zgf�T$j_a�X[XgT_l(U�ac^�~�ik^ $ lpTVZHacb�l[io\ U }kT�Xga?a�Zgl�� 5Fa�XHT_W�j f�*�+%�y6}kTVZ-,/. = T$Z[f�T'j_a�X[X[T_lpU�ac^�~�io^ $ XgT8Z T_j Zgikac^W�^�~�� Zgf�T l[� = $ Xgac�U�acb ��021bW ^ Zgf�T > ~�T�7 ^�T�y W�^�~435.1W]Xgaza�Z�n`TVjVZga�X,acb'� T_i $ f�Z6*A�I� b$jVac�Xgl[T� ik~�T_^`Zgi37 T_l �)ioZgf����.�7�V�8� � W�^�~<� T�l[ik\sU } > ��l[T��,�8� Zga W�}kl[as~�T_^�a�ZgT W�XgTVU�XgTVl[T_^`Z�WYZgion`T ik^���&�������O#���,� �)T_j W�}k} � U�y U'f WYU/�A=��Zgf�T6L&Xg��f WYZ ~�TVj_ac\ U�aclpioZgioac^

�,� 9:<;�=

�����?>

m�l[io^ $ = WYX Zga ~�T_^�a�Z[[email protected] !i�j_}oacl[�XgT ac^�T�f W�l����� � 9

B�CD:��E��F�

�)f�TVXgTAG ~�T_^�a�ZgT_l+Zgf�TbL&Xg��f WYZ a�Xg~�TVX:� � f�T6L&Xg��f WYZ j_TV}k}IH : �������$� � io^ ��� � f W�l W�l[ik\ ik} WYXj_}oacl[�XgT,U�Xga�U�TVXpZ > � f $ W�ik^L~�io\ H : �(JN���6�gy � f�T XgT"JN�8K ��~�T_^�a�ZgT_l+XgT_~���j_TV~M}kT_^ $ Zgf,��{|^�UFWYXpZgikj_��}kWYX~�ik\ ��� � �MLN! ) L �� PO'�� W!`T�*%�+% ���)T_j W�}o}M� U�y 3�Xga�U/�A=��>; ���N��Zgf WYZ

�A,Q.I����� ���A,Q. ���?� cSR ,/.'�UT/����V,Q.I����� � �)���� c�R ,Q. �XW/��>

5Fa�XhT W�j�f�* �Y%�y+}kT Z�_Z. ~�T_^�a�ZgT Z[f�T UFWYX�W = ac}oikj l[� = $ X[ac�U i Zh~�T87 ^�T_l�� � ^�Tef W�l�_Z. ��A,/.5� �[�A,Q.I��\�� � ��T ZV] . �[� c�� _Z.��1{|Z[ls^�io}oX�W�~�ioj W�}_^�. f W�l jVaz~�ik\ T_^�l[ikac^ t io^� ��J TVZ

ha` h

Page 6: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

� .#~�T_^�a�ZgT Zgf�T l[T Z,acb$W�}o}�L a�XgT_}2l[� = $ Xgac�U l�j_ac^`Z�W�ik^�T_~eio^ _ .+��{�Z,ik~�T_^`Zgi�7 TVl.�)ioZ[f _Z.D� � W�^�~ ioliklpac\]a�X[U f�ikj�Zga Zgf�T�U�X[a�|TVjVZgion`T�}kio^�T�� � �

5Fa�X�T W�j�f.� ��� yF}kT Z : � � X[T_lpU/� : �� ~�T_^�a�ZgT�Zgf�T#�"j_ac^|� $ WYZ[T acb�� � XgT_l(U/� : �� W�^�~MlpTVZ� : � : � + �H�b� ^�T�f W�l�� c��A� : � �+ : #� {|^ UFWYX[Z[ikj_��}kWYX"������iol Zgf�T ��^�ioU.a�ZgT_^`Z�X�W�~�ikj_W�}Pacb_Z.�W�^�~- #+ ��� � ^ .��O#��� � ��TVZ�� � � � f�ioj f�ikl,Zgf�T j_ac^�T acb$^�ik}oU.a�ZgT_^`Z TV}kT_\]TV^�Zgl�acb Q �\f�b ZgT X S > ^�!zik^�y��X� � iol7 ^�ioZ[TN� �'W�j�f�( �� � � iol)io~�T_^`Zgi�7 T_~ � i Zgf W �;a�X = i Z io^� y�j W�}k}oT_~ W ^�ik} U�a�ZgT_^`Z�a�X = ioZ:�

5 i � ( ��� � �F��f�b ZgT X&�`UFW�}oZgT_^�l(ZgT_ik^�y:( +� �iol T�*z��io~�ik\]TV^�l[ikac^ W�}/� �Ft���acbF~�ik\]TV^�l[ikac^ �� ~�io\!(���&W�j�fMj_ac\sU�ac^�T_^`Z� acb ( +; �iol�j W�}k}oT_~eW�^ a�X = i Z�W�}�n�WYXgioTVZ > �4W�l[l[azjVi WYZgT_~BZga-(F��� � f�T > f Wvn`T�Zgf�Tb ac}k}oa"�)ik^ $ XgT_\hWYX !�W = }oT�~�TVl[jVXgi U�Zgikac^ ~���T�Zga��`Z[T_ik^ = TVX $ y�l[TVT � � ;2� b a�X)T�� W�\sU }kTN�

5 i ��� �.� � � f�T_^ �g� + : �Hiol$i X[XgT_~���j_i = }kT W�^�~hWb7 ^�i ZgT)��^�ioac^]acb�^�io}oU�a�Z[T_^�Z$a�X = i Zgl_y`f�T_^�j_TW�~�\]i Zgl Ws��^�i *?��T�~�TV^�l[T a�X = i Z (S���6���&��TVZ� ?���6��� (S���6� + ��� �+ : �� ��{�Z iol W�^Ba�X = ioZ�W�}�nYWYXgikT Z >W�^�~ T n`TVX > a�X = ioZgW�}�n�WYXgioTVZ > l[a a = Z�W�io^�l�� � f�T�\hWYU�� 1! ���� (S���6�?� XgT_lpU/��� 1 a� ����� ?���6� �'iolj W�}o}kT_~MZgf�T]�`ZgT_io^ = TVX $ � XgT_l(U/�,X[i $ f`Z��`Z[T_ik^ = TVX $ � \hWYU/�G� = lpTVX[n`T�Zgf WYZ ����� 1 &�� � (g� iob$W�^�~ ac^�} >ikb�( + #+ : �ikl ~�T_^�l[T io^- #+ : ��{|^ UFWYX[Z[ikj_��}kWYX � ��� 1 &��� (F����=��& � ��� 1 &��N� (g���O#���#� %,ioT8� �! �� W�l)W � lpTVZ ��^�~�T X�~�i W $ ac^ W�}�W�j Zgikac^BW�^�~ l[TVZ#" : � �F���?� : ��� �)f�ikj�f ikl W �a�X = i Z io^+�$ +� � � fX[ac� $ f`L&Xg��f WYZ ~�TVj_ac\ U�aclpioZgioac^Bac^�T�f W�l

�% �1� 9: ; =

" :

W�^�~

�'&�� (" : � 9B�CD:" B >

{|^BUFWYX[Z[ikj_��}kWYX)Zgf�T)" : WYXgT�}ka?j W�}k} > j_}kacl[TV~ ik^�Zgf�[email protected] !i�Zga�U.ac}ka $ > �O#�+*,� ��TVZ,� � - � 3 ��� � ��� � �.��� �. � 3 � � c�� � � + � c�� � �0/ �8{�ZLiklLj W�}k}oT_~ Zgf�T�`ZgT_io^ = TVX $ nYWYXgioTVZ > � J TVZ�%21 c � t � ;���r = T�Z[f�T�U�XgaN|T_jVZ[ikac^ acbPZ[f�TGc�)4Zgf j_ac\ U.ac^�T_^`Z acb3��W�^�~ lpTVZ

h54kh

Page 7: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

%8�(% � � %�� �(% � ?%��Y�+��io\]ik}kWYXg} > ycZgf�T,j_a�ZgW�^ $ T_^�Z = ��^�~�}oT�� ��� ^ ���M��� � ��. hacb � ik~�TV^�Zgi37 T_l�)ioZgf

- � 3 ��� � � �� ��6L 3.�+� c���� � / >{|^ Zgf�iolhU�XgT_l[T_^`Z�WYZgioac^ Zgf�TeU�XgaN|T_jVZ[ikac^ acb�0Zga1Zgf�T`7�XglpZBbDW�jVZga�X�io~�T_^�Z[i�7 T_l � i Zgf Z[f�T \ ac\]T_^`Z\hWYU/�h{|^#Zgf�ikl�bDW�l[f�ikac^!� ik~�TV^�Zgi37 T_lI�)ioZgf Z[f�TS7 = XgT U�Xga?~���jVZ� ����6�-5Fa�X T_W�j f � � � yHlpTVZ

� : �(%�� &�� � " : � �P{�ZPiolHW�^ i X[XgT_~���j_i = }kT&}oazj W�}o} > j_}oacl[T_~ l[� = lpTVZ�acb �Bacb~�ik\ T_^�l[ikac^R; O]W�^�~ ik~�TV^�Zgi37 T_l�)ioZgf#Zgf�ThjVac^�a�Xg\hW�}�Zga " : io^ � ^ ���� '�M��� � f?��l Zgf�T�(� : � �!� WYX[TG|��lpZ Zgf�ThioXpXgT_~���j_i = }kTj_ac\ U.ac^�T_^`Zgl)acb��T� f�ioj f ikl f�T_^�j_T T�*?��ik~�io\]T_^�l[ioac^ W�}]�O#���#� 5�a�X T W�j�f (@� �X� ��y.l[TVZ �M� (F����% & �� (F��� {�Z�ikl�Ws}kazj_W�}k} > j_}oacl[T_~el[� = l[T Z,acb ~�ik\]T_^�lpikac^; O'� ��T Z � : � (F� � � � (F� + � : � � f�T_ioX ~�ikl�|acio^�Z ��^�ikac^ iol�� � (F���-� ^�T f W�l �,��� 1 &��� (F��y�iobHW�^�~ac^�} > iob � : � (g��� (� : � ��ik^�jVT � � (F�'iol+T�*z��io~�ik\]T_^�lpikac^ W�} � � ;2��Zgf�T�}kWYZ[ZgTVX WYX[TM|��l(Z)Zgf�T�ioXpXgT_~���j_i = }kTj_ac\ U.ac^�T_^`Zgl)acb �M� (F� �O#���,� ��TVZ�� ��- � 3 ��� � � � � �� 3.� � c�� � � / W�^�~ }kT Z�% 1 �c�� t � ; = T,Zgf�T,U�XgaN|T_jVZgioac^ acb Zgf�Tc�)4ZgfMj_ac\ U.ac^�T_^�Z�acb��S�?��b�j_ac�XglpT� ik~�T_^`Zgi37 T_l?� i Zgf+% &��� ����� & �A�&{|^ Zgf�ikl \hW�^�^�TVX�� b �XpZgf�TVXik~�TV^�Zgi37 T_l �)ioZ[fBZgf�Tb7 = XgT�U�Xga?~���jVZ��� �� �

5Fa�X,T W�j�f�� �%� y�lpTVZ�� : �U% & �� �7H : � ��{�Z,ikl,W�^ ioXpXgT_~���j_i = }kT�}ka?j W�}k} > j_}oacl[T_~ l[� = l[TVZ�acb��S�{�Z io~�T_^`Zgi�7 T_l6� i Zgf�Zgf�T j_ac^�a�Xg\]W�} - ���� )+ : � ��� ����� ���� � / acb ����� io^�� ^ ���M��W�^�~#lpa f W�l~�ik\ T_^�l[ikac^ O'� � f?��l Zgf�T (� : �� � � WYXgTG|��lpZ�Zgf�TLioXpXgT_~���j_i = }kTLj_ac\ U.ac^�T_^�Z[l acb�� �)f�ikj�f iolf�T_^�j_T T�*z��io~�ik\]T_^�lpikac^ W�}]� ��T�j W�}o}��;Zgf�T jVac^�a�Xg\hW�}�nYWYXgikTVZ > �

�,� � RP��� ��NP��J6I�CFA&@�C�J�G �?� � �

�,����� ��T�ik^`n`T_lpZ[i $ WYZgT�l[ac\]T,U�Xga�U�T X[ZgikTVl acb2Z[f�T j_ac^�a�Xg\]W�}�n�WYXgioTVZ > � ~�T87 ^�T_~Mio^`; �C=��

�,�PO#� {�Z$ikl�j_ac^`n`T_^�ikT_^`Z&Zga�ik~�T_^`Zgiob > � � i Zgf ���<���S�-& �#W�^�~ � : � i Zgf �!� : ���S� & � :#"%$ �AU&}oT WYXg} >� �OXgT_lpU/��� �'ikl�� � XgT_l(U/� �R�'lpZ�W = }oT ��^�~�TVX�~�ikW $ ac^ W�}�W�jVZgioac^MW�^�~�� � �� � � � �_��� : "&$ ��� : � �. '�(� : �$dea�X[T_a�n`T X (� :#"%$ � � (� : � �. �� (� : l[io^�j_T ��� � ikl j_ac\ U }oTVZgTN��,���,� 5Fa�X]T W�j�f }oazj W�}o} > j_}oacl[T_~ l[� = n�WYX[ikTVZ >*) acb l[TVZ+� � ) � � % &��N� ) ��� L > ~�T87 ^�i Zgikac^

h-, h

Page 8: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

� � � � ����!5�a�X]T W�j�f ( � � � �A��� � �;y+l[TVZ � : � +�(F�;� � : +�� � +(F��� {(~�TV^�Zgiob >� : � )+�(F�?�)ioZgf ��� : � +�(F�������)ik^�� : "&$ � (F��� � f�T_^ � 1 � �.�!� : � �+8(g� ���S� �M���&����� �M�W�^�~ �F��� : � #+ (F������� ��� :#"&$ � (F��� U'ac^�l[T�*z��TV^�Zg} >�'&�� ~�io\ � : � �+ (g��� ~�ik\ � : "&$ � (F� � O->�'�zI�I�C � �������� ( �� � � �� ��������

����� � � #+-(g� � 9:<;�=

� : � �+�(F��>

������� � � #+-(g� � 9:<;���� "&$��� "! � : � �+�(F�$#�����%�&� � � #+-(g� � ���')( �+*,��- �. /� � �� 0���1 ��2 � � �3��� * (54 �+6 7�84�� -:9 �, ��. 2 � � : � �+�(F�<

�,��� 1 &�� � (g� �<; *,�%- �. �� � �� O=#> Jv��� � � �4iP��iol�Z[Xgi nikW�}]� 5Fa�X��4ikiP��a = lpTVX[n`T]Zgf WYZ ��� �+�(F��W�^�~(� : � �+�(g��WYXgT?�0lpZgW = }kThW�^�~XgT_j_W�}k}�Zgf WYZ���� � iol)j_ac\sU }kTVZgT� � f���l

�g� � � �+�(F� ��� �� � #+-(g� �� �M� (g���� ?:<;���� "&$ �� "! � : � (g� � =?> ;�> � �

� �A@B ?: ;���� "%$ �C "! � : � �+�(F�<DEX>

��ik^�jVT]W�}k} Zgf�TVl[T l[T Zgl WYXgTV� lpZgW = }kT]W�^�~ ���&����� �M� ac^�Tsa = Z�W�ik^�lg�4ikiP� =?> Z�W!io^ $ io^�Z[TVXgl[T_j Zgikac^�)ioZgf % &��� ��������)T_j W�}k}�Zgf WYZ � � � 1 &��N� (F�sikb W�^�~ ac^�} > iob� + : +�( iklL~�T_^�l[T ik^ + : � � f?��lBikb

� ��� 1 & � � (F�+ioZ,b ac}o}ka � l)Zgf WYZ�� : � �+.(g���U��� #+ : + ($� ����� iol�ioX[XgTV~���j_i = }oTN��dea�X[T_a�n`T X,ik^Zgf�iol)j W�lpT6� T�a = Z�W�ik^Bb Xgac\ �'&��'Z[f WYZ

~�io\ � : � #+-(g� � ~�ik\ � : "&$ � (F� � O8�(O->hGFkh

Page 9: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

q�T_^�j_T��4ikioi ���

�,��� � ��io^�j_T (� :�� � : � ( + �&iol)i X[XgT_~���j_i = }kT acb2~�ik\]TV^�l[ikac^ O y �&T�a = Z�W�io^BZgf�T� ��Jv�.ADA CFJv� � � ��� (� : �,��� 1 &��N� (F� �,� � 2 ��� � �3��� * (54 �+6 7�43� -:9 �� �. 2 � �<; � � �+�(F�)#�,���#� ��TVZ

���g���6����� 1 &�� �'� 18� �6� ��� ���g� �6�<��� 1 &�� � � 1����6� ��>

� f�T_l[ThWYXgT l[� = l[TVZgl acb>� j W�}o}kT_~�Zgf�T $ T_ac\]T Z[Xgikj ~�ac� = }kT j_T_}k}olS� X[T_lpU/�Lj_ac^�T_l�� � f�l6� T]lpf W�}k}�l[T_T���g� �6�)^ WYZg�X�W�}k} > ~�T87 ^�T_l WLZ�� a)�l[ik~�TV~ ik~�T_W�}Hik^eZgf�T $ Xgac�U#W�} $ T = X�W;�#� � dMa�XgT_avn`TVXb� T�l[f�a �Zgf WYZ�Z[f�T XgT_l[��} Zgik^ $ � �� \]az~���}oT lpZ[X[��jVZg�XgT�~�TVZgT Xg\]ik^�TVl Zgf�T b a�Xg\0acb � 1����6� �b� = l[T X[n`T Zgf WYZ���g� �6�<�����F����&�� �����TVZ

�g���6����� 1(a &�� � � 1(a�� �6� ��� �g� �6��� � 1(a &�� � � 1 a ���6� ��>� f�T_l[TMWYXgT�l[� = l[T Zgl]acb � j W�}o}kT_~�Zgf�TBXgi $ f�Z $ T_ac\]T Z[Xgikj�j_T_}o}kl � XgT_l(U/��j_ac^�T_l ��� f�l\� T l[f W�}o}+l[T_T�g� �6� ^ WYZg�X�W�}o} > ~�T87 ^�TVl W#Xgi $ f�ZLio~�T W�}�acb �#� � dMa�XgT_avn`TVX�� Tel[f�a �0Zgf WYZhZgf�T X[T_l[��}oZ[ik^ $ �\]a?~���}kT l(Z[Xg��jVZg�X[T ~�T ZgTVXg\]io^�T_l Zgf�T b a�X[\ acb � 1 a ���6� � � fXgac� $ fMZgf�T ~�ik\]TV^�l[ikac^ TVlpZgik\]WYZgT io^�; ���ac^�T�f W�l��g���6�>�����g���b� + �g���b��� � fXgac� $ feZgf�T]�`ZgT_io^ = TVX $ nYWYXgioTVZ > ac^�T�j f�TVj�!zl�� l[T_T �1 `ry�u�� x2�b a�X)T�� W�\ U }oT2��Zgf WYZ

�'&�� ���g���b� � 9B�;=� :'"&$ ! �g� E���>

�'�zI�I�C � � ��� � � 43� E ���F����&�� � � 2 ���.� � ��� � � 2 � � �3( 6 � � 2 � B & ���g���b� �3(543� 2 � � 2 2 ���(���� �� ��� B �,� � 2 ��� � �3� � * (/4 �+6 7� 4�� -:9 �� ��) 2 � �<; � � ?� E�� �)#> Jv��� � � J2TVZ � � ?�7E�� � 1 c*� t � ;�� K K K�� , B y = T Zgf�T i X[XgT_~���jVi = }kT�j_ac\sU�ac^�T_^`Zgl,acb � � ?�7E � � �.J TVZ( � (S���6��� L > �'&��$ac^�T f W�l

�+�( � ?B�;=� :#"%$ ! ?�7E ��>h��kh

Page 10: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

{�Z b ac}o}ka � l+Zgf WYZ� � �+�(F� � ?

B�;=� :'"&$ !���?1 � �� � ?�7E � � 1

� ?� ;�� � : ! (��� � =?> r >�� >

� f�T_^�Z[f�T,W�l[l[TVX[Z[ikac^Lb ac}k}oa"�)l =�> ��^�i *?��T_^�T_l[l acb�~�T_j_ac\ U.acl[ioZgioac^hik^`Zga ioXpXgT_~���j_i = }kT,j_ac\ U.ac^�T_^�Z[l��

�,�+*,� {�Z&b ac}k}oa"�)l&io^ UFWYX[Zgioj_��} WYX b X[ac\ r�� x,Zgf WYZ � � �7E�� �Hikl$T�*z��io~�ik\]T_^�lpikac^ W�}]��dMa�XgT)U�XgT_j_iol[T_} > � Ta = Z�W�io^BZgf�T

> Jv������� G ��GD��N � � ��� � � �,�.� ������ � ��� � �3��� * (54 �+6 7� 43� -:9 �� �. 2 ��<; � � ?���6� � �,��� 2 ��� (� B PE � �g� �6�$#������� � ��� � �3��� * (54 �+6 7� 43� -:9 �� �. 2 ��<; � � ?���6� � �,��� 2 ��� (� B PE ���g� �6�$#> Jv��� � � U'ac^�l[ik~�TVXS�4ikiP��� ��ik^�jVT�9ikl,U�XgaN|T_jVZgi n`T�y % ikl�WBj_}oacl[T_~#\]WYU/� q�T_^�j_T$% � (� B � ikl�j_}kacl[TV~�y

j_ac^`Z�W�ik^�l ��� �+ B �&W�^�~ ikl j_ac^`Z�W�ik^�T_~ ik^ ��� + B �� � � f?��l-� T a = ZgW�ik^�'&�� % � (� B �<� ?� E�� >

4�a � =?> r�� x ?B�; ��

(� B � � � ?���b� �<&�� � ?���6� ���W�^�~ l[a

?�7E �<& % �!�S� ?���b� � ��� ?���6� >L > ~�T87 ^�ioZgioac^ Zgf�iol $ i n`T_l_E � �F���6�HW�^�~]l[a � : & �g� �6��+ ���g���6� � �g� �6���A� ^ Zgf�T a�Z[f�TVX&f W�^�~ ?�7E ��� ?���6��y b a�X W�}k} E;���F���6�&W�^�~ l[io^�j_T � :���zy�Zgf�ikl b a�Xgj_T_l � :�� �F���6����q�T_^�j_T��4ikiP���� f�T�U�Xga?acbPacbM� i �$ikl l[ik\ ik} WYX�� 5 i XglpZ)acbHW�}o}

� � �+ ( � �6� ��� ?B�; � =� : ! �S� #+-( �7E � � �

� ?B�; � =� : ! (� B � � ELr >�� >h��� h

Page 11: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

�'TVZ6 )+ ( �7E �*��� � ; =� B "&$ ! ?� �e� W�^�~el[aLZgf�TVXgT T��iklpZ[l,WLlp� = l[TVZ�� : & ���F���6� l[��j�f�Z[f WYZ�Zgf�T ?� �e� ��� � : WYXgT�Z[f�T�i X[XgT_~���j_i = }kT jVac\ U�ac^�T_^`Zgl acb �+ (S���6� �+��io^�j_T

~�ik\ �7E���� t; ~�ik\ ( � �6� ikb E;� ���F���6�W�^�~

~�io\ �7E��<W t; ~�ik\ ( ���b� ikb E � ���F���6��� ���g���6�� T \]W > ~�T_~���j_T Z[f WYZ � : � ���g���6���8�G� � f�T_^eW�l ik^Mr����]��^�i *?��T_^�T_l[l�acb ~�T_jVac\ U�acl[i Zgikac^Bik^`ZgaioXpXgT_~���j_i = }kThj_ac\ U.ac^�T_^`Zgl $ ion`T_l Welp� = l[TVZ (� : & ���g���b��l[��j�f�Zgf WYZ Zgf�T (� B DE� (� : WYX[T]Zgf�TioXpXgT_~���j_i = }kT j_ac\sU�ac^�T_^`Zgl�acb�� � ?���6� � � � fXgac� $ fT� &��A� T j_ac^�j_}k��~�T�W�l)ik^H� i �+W�^�~ Z[f�T ~�T87 ^�i Zgikac^acb �F���6��y�Zgf WYZ � : � �g���6� ��,���#� ��T�lpT_T,b Xgac\ r�� � Zgf WYZ � � ?���6� �$W�^�~ � � ?���6� ��WYXgT = a�ZgfLT�*?��ik~�ik\ T_^�l[ikac^ W�}�acb�~�ik\]T_^�lpikac^O'� � ^�T&\hW > XgT_\hWYX !�Zgf WYZ ?���6��ikl = W�~�} > = T_f Wvn`TV~ ik^ Zgf WYZPioZHiolP^�a�Z io^ $ T_^�TVXgW�}W ��^�ioac^ acb�a�X = ioZ�W�}nYWYXgikTVZ[ikT_l_yFl[T_T<� � �2W�^�~ � d1y � � ; � x2�E�&�(� ^ Zgf�T a�Z[f�TVX�f W�^�~ =�> �1 `ry�� ���N�2W�^ > j_}oacl[T_~MioXpXgT_~���j_i = }kT��lpZ�W = }oT�ik^`n`ac}k�Zgi n`T�l[� = n�WYXgioTVZ > acb hikl&Zgf�T�jV}kacl[�XgT,acb2W�^La�X = ioZ�W�}Fn�WYX[ikTVZ > j_a�XpXgT_lpU.ac^�~�ik^ $ Zga Zgf�T��^�i *?��T ~�T_^�lpT a�X = ioZ io^ � -� ��{|^BZ[f�ikl lpT_^�l[T � � ?���6� �'ikl = T Z[ZgTVX:���io^�j_T � � ?���6� � � %�&��2� ?� �6� � � % &�� % � (� : �gy-� Tej W�^ a = Z�W�io^ b Xgac\ r�� ��W�f�ac\]ac}oa $ ikj W�}

WYU�U�Xga`W�j�f Zga�Zgf�T ~�T_l[jVXgi U�Zgikac^ acb �g� �6��� ��ik\]io} WYXg} > % &�� % � (� : �8� % &��2� (S���6� �8� � � (S���6� ���� f�T_^ ; � � $ ion`T_l)Wsf�ac\ ac}ka $ ikj_W�}�WYU�U�Xga`W�j�f Zga Zgf�T�~�TVl[jVXgi U�Zgikac^ acb ���g���6� �

� � ���?� � RP�?�.Jv�?��GDQ � �.N F�.AP�2��G ��N �

� ����� J2TVZ�H = T W]l[T Z:� f�l ik^ �CU-@ y#; � � � x2�/� T \hW > ~�T�7 ^�TsWsj_ac^�n`ac}o�Zgikac^ U�X[az~���jVZ�ac^Mlp� = l[TVZglacb_H H W�l�b ac}k}ka �)l��gJ TVZ��21 � H H H � H H = T�Zgf�TsU�XgaN|T_j Zgikac^�ac^�Zga�Zgf�TFc � ��)4ZgfbDW�jVZga�X�� @�ion`T_^ �<� "�& H �H1y l[TVZ

����" ��� � � ��� & �� � ���g� +�� &��� � �'"&� ��>��T j W�}k}�i Z�Z[f�T j_ac^`n`ac}k�Zgioac^�U�Xga?~���jVZ�acb � �)ioZgf "b��{|Z�ikl�W�l[l[a?j_i WYZ[ion`T 2 = �Z ^�a�Z�j_ac\]\��Z�WYZ[ion`TN�U'}kT WYXg} > Zgf�T ~�ikW $ ac^ W�}D! & H �H iol Z[f�T ik~�TV^�Zgi Z > b a�X Zgf�iol U�Xga?~���jVZ:�

h�� �Bh

Page 12: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

� �PO#� ���X,io^�Z[TVXgT_lpZ�io^ Zgf�T j_ac^`n`ac}k�Zgioac^eU�X[az~���jVZ,j_ac\ T_l�b Xgac\ Z[f�T b ac}k}ka �)ik^ $ T W�l > = �Z,jVXg��j_ikW�}XgT_lp��}oZ:� � W!`T H ��� � �?^ ���M��W�^�~�nzikT8� � � � W�l j_}oacl[T_~ lp� = l[TVZglhacb�� �6� � T_j_W�}k}+Zgf�T\]ac\]TV^�Z \hWYU %� �?^e���M� � �k�

�'�zI�I�C � ������ �� � & � � � �3� 9 # �Y&�� � �, �� � �������� � �A� � � �(% &�� % �]�G�������� � � �,��% &�� % � �R������%�&� � ���=; ��� - (� � � ��� ��� �. '� � * ��� ��.� � �� ���� 2 � �� �� 4��,�3� �.� 9 �� * �. �4�� 6 � 2 1 �3�. �� �� �,� � � 2 �3( 6 � � 2 � � & � �, * � � �� �,� � � 2 �3( 6 � � 2 � � ; � #�� ; �a& � � � � � ��,� � � �, 2�, * �Y&�� � � � � � ��,� � � �, 2 � �� �� � ���

� � ��� � �F�]� � �R� >

� 9 �,� 2 � 4.(� � � �� �?� ���R� � �g�7% &�� % � �I� ��>

��� �3�� �.�8� � 2 ��� � 6 �� � � � 2 �8�,� � 4)7� � � * � ; � � �84$7� � � * � � � �, * � � � 4)7� � � * � � �, * � � �� 2 � 6 7� #> Jv��� � � �)T_j W�}o} Zgf WYZ���� � �� � � - � 1 � � 1 � �*� � � % � 1 � � � % � 1 � � / � � f���l�% & � % �]�G� �- ��, � 1 �)� � � �0 � � � � � � % ��,���a% �0 � ��� % �0 � ���a% � 1�� / ����^�Zgf�TLa�Zgf�TVX�f W�^�~9��,P� 1 �)����.� � � \]T W�^�l Zgf WYZF� T�j W�^K7 ^�~9�0 � � � � � � l[��j�f Zgf WYZ$,�� % & � % �0 � � � 1 � % & � % �0 � ���U'ac\ UFWYXgiklpac^ �)ioZgf�Zgf�T�U�X[TVnioac��l)b a�Xg\���}kW $ ion`TVlG�4iP���

�)T_j W�}k}HZ[f WYZ�� � - � 1 � 3 �>� � % � 1 �*�U3 / � � f���l6%�&�� % � �R�*� - ��, � 3 �>� � � � 1 � 3/�6��j� i Zgf % ��,� � 3 / � ��^ Z[f�T a�Zgf�TVX�f W�^�~ ��, � 3 �6�%� � � \ T W�^�l�Z[f WYZ6� Tsj W�^�7 ^�~0� 1 � 3/� � �l[��j�f Zgf WYZ�,��+% &�� % � 1 ��� q�TV^�j_TS�4ioi �����io^�j_T � � � � �]yZ[f�T W�l[lpi $ ^�\]T_^`Z-���� �1++�;b Xgac\ Zgf�T,l[TVZ)acb � io^�nYWYXgikW�^�Z)l[� = l[TVZ[l acb

�1Zga Zgf�T�lpTVZ�acb �I)�ik^`n�WYX[i W�^`Z l[� = lpTVZgl�acb�� ikl ik^`n`TVXgl[T�ZgasZ[f�T�\hWYU � �� ���F�

���U�U.acl[TI� � - �� � � � � / & � W�^�~ �,� - � 1 � 3 � / &��S� � f�TV^

h��Yi h

Page 13: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

� � �,����� �0 � � 3/� ikb � � 1 �

a�Zgf�TVX �)ikl[T >

�)T_j W�}k}�Zgf WYZ�Zgf�T $ X[ac�U � W�jVZgl ~�ikW $ ac^ W�}k} > ac^ � �G� � f�T W = avn`Tsb a�X[\ ��}kWBio\ U }kioT_l�Zgf WYZb a�X W�^ > � � �Y�#�]� � �I� �(�#�]�G� �<� � �R��� � f�TVXgT_b a�XgTYy

�F�]� � �R� � � � ���

ikb � iol�� ik^`n�WYX[i W�^`Z:�

{|^hUFWYX[Zgioj_��} WYX�� � � � �I�<� �F�'� � �R��ylpa�Z[f�T�} W�lpZ+b a�Xg\���}kW acb�Z[f�T�}oT_\]\hW�b ac}k}oa"�)l b Xgac\ �4ioi ���

� f�T } W�lpZ�j_} W�io\ acb Zgf�T }oT_\]\hW b ac}k}oa"�)l�b Xgac\ Z[f�T bDW�jVZ�Zgf WYZ�%�iol�W]j_}oacl[T_~ej_ac^`Zgik^���ac��l,\hWYUW�^�~ ��� � ikl j_ac\ U }oTVZgTN��h�I�CFJ�� � 4�a�Z[T�Zgf WYZ Zgf�T�j_a�X[XgT_l(U�ac^�~�T_^�j_T ~�T_l[jVXgi = T_~Mio^V�4ioikiP�&W�lpl[i $ ^�l#� : Zga � :'"&$ �� ���,� {�Z)ikl ik^�lpZ[X[��jVZgion`T,ZgaF7�XglpZ�\]W!`T�lpac\]T�jVac\ U �Z�WYZgioac^�l �)ioZ[f �9XgTVU }kW�j_T_~ =�> � ��'�zI�I�C � � ����� 3 � E;�8� �3(543� 2 � � 2 J�� 3 ��� JN�7E�� � J�� 3 E��)# � ���. " � " B � " B # �������� �)�2 ��� � 2 �.�3� ��4 2 � �� � &��� � �'"�� +�� &��� � �'" B � � � 2 � �, �� � �.��� � �, * 2 ��� � �3� 2 � � 4 2 � ��

� � � � &��� � �'"� � + � &��� � �'" B � � � "� B� � � 6.� � � 2 � �, 0� � � � - ��� 9 � � � - #> Jv��� � � ��^�T f W�l � &��� � �'"� � � - � � �?� � 3I�?��� �3� � � ������ � � / W�^�~ � &��� � �'" B � �- ��� � � � � � �?� � � E � / � � � �A��� � � � � / ��4�a � � � �?� � 3I�?��� ���)� ��� � � � � � �?� � � E ����ik\ U }oikT_l�Zgf WYZ� & � � � � 3���y�b a�X l[ac\]T�� � � � � f�T_^�Zgf�ikl+j_ac\]\ ac^�TV}kT_\]TV^�Z T�*?� W�}kl � �7�F� 35�?� 3�� E ��� � f�ioj�f f W�lik\]W $ T6�#���?� 3�� E ������^�~�TVX � � �e� ��io^�j_T�JN� 3/��� JN�7E �<�(J�� 35E �'io\ U }kioT_l+Zgf WYZ���35��E�� � ��3 E � � Tj_ac^�j_}o��~�T Zgf WYZ�"�� 0" B ��"� B �

@�ion`T_^.� � �;y.l[T Z �M���b��� -/* � !V) ��* � ! & / ��m�^�~�TVX�Zgf�T f > U.a�Zgf�T_l[iol�Zgf WYZ�}oT_^ $ Zgf�lW�~�~ i Z)b ac}o}ka � l+Zgf WYZ

� � � 3 E�� &�� � � 3 � �7E &�� � \�� � 3 &�� ��>

h�� ? h

Page 14: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

� f?��l � c��Y� 3 E � & � 3 E�� &�� + ���.iol�Zgf�T$~�i XgT_jVZ l[��\8acb � c�� 3��7E�� &�E'&�� +Z����3 &���W�^�~ � c��Y� 35� &�3 &�� +����� � ^ Zgf�T�a�Z[f�TVXef W�^�~ b a�X�W�^ > � � � Zgf�T�\���} ZgioU }oikj WYZgioac^ \hWYU8io^�~���j_T_l�W�^ iol[ac\]a�X )U f�iklp\ ����� & � &�� + ���� %���$� � ac^`Zga'�����$� � W�^�~�Z[f�TBZ�W�^ $ T_^`Z]l(UFW�j_TMWYZ����$� � ikl.|��lpZ� c�� � ��� &<��& � + �����1{|Z b ac}k}oa"�)l Z[f WYZsZgf�TB\���}oZgi U }kikj_WYZgikac^�\hWYU io^�~���j_T_lhW�^�ikl[ac\]a�XpU f�ikl[\ acb� 35� &-3 &���+ ��� 3+�7E � &�E'&�� +F�V��3 &��,ac^`ZgaV3 E � & � 3 E�� &��<+F� W�^�~ f�T_^�j_T W�^ ikl[ac\ a�X[U f�ikl[\acbZ��35� +��E�� ac^`Zga��3 E � �4W�l)iol-� TV}k}3)]!^�a � ^#� �

4�a ��XgT_j W�}o}�Zgf WYZLZgf�T \���}oZ[ioU }kioj WYZgioac^ \]WYU io^�~���j_T_l W�^ ikl[ac\]a�XpU f�ikl[\ acb � &. � ac^`ZgaioZ[l io\hW $ TF� & � �)f�ikj�f�iolsa�U.T_^�~�T_^�lpTBio^ �F� � f�T_^ � & ���?� 35�?����35��E ��� ikl�a�U�T_^ ~�T_^�l[TBik^� &��� � � "� �<+ � & �� � �'" B ��W�^�~ iol[ac\]a�X[U f�ioj�Zga'� & ���?����3I�?����35��E���� �)f�ikj�f \hWYU ls��^�~�TVX � � �Zga � &% K�7�F����35��E �����?L > Z[f�T�X[T_l[��}oZ,acb�Zgf�T UFWYX�W $ X�WYU f�W = avn`T Zgf�iol,X[T_lpZ[XgiojVZgioac^ acb � � � ikl�W�^iklpac\]a�X[U f�iklp\ �

5 io^ W�}k} > Zgf�T ZgW�^ $ T_^�Z,l(UFW�j_T Zga Zgf�T U.acik^`Z\���?� 35�F� 3�� E�����ik^ � &��� � � "� ���� "� � �OXgT_lpU/�)ik^� &��� � � " B � �� � !" B � iol =�> Zgf�T W = a�n`T�iklpac\]a�X[U f�ikjhZga8 & " & +. ;" �� B & � XgT_lpU/�% & " & " �� � B & +� �� ��� {|Z[l)io^�ZgT Xgl[T_jVZ[ikac^ iol* & " � & +� � " �� � B & +� � �)f�ikj�f f W�l ~�ik\]T_^�lpikac^O � JN� 3/� �(JN�7E ���1��io^�j_TLZgf�ikl�T�*?� W�}kl�~�ik\ � &��� � �'"�� � ~�ik\ � &��� � �'" B � � ~�ik\ � � �)y�Zgf�Tik^`ZgTVX[l[T_jVZgioac^ � &��� � � "� � + � & �� � �'" B �&iol Z[XgW�^�lpn`TVXglgW�}��� ��� � � T_j_W�}k}&Zgf WYZ � �ikl?|��lpZ Zgf�TBjVac^�a�Xg\hW�}&Zga "�1io^ � ^ ���� � � � � f�T_^ =?> �CU-@ y ; � � � ;���WYU�U }kioT_~ Zga � � r a�X ~�i XgT_jVZ[} > � T�a = Z�W�ik^

� ��Jv�.ADA CFJv� � � ����� 3 � E��.� �.(/43� 2 � � 2 J�� 3 � � J��7E ��� JN� 35E �$# � ���. � � � B ��� B # �������� �)�2 ��� � 2 �.�3� ��4 2 � �� � &��� � �'� � + � & �� � �'� B � � � 2 � �, /� � �.�3�$� �, * 2 ��� � �3� 2 � � 4 2 � �� � � � � &��� � �'� �� +�� &��� � �'� B � � � � B

� � � 6.� � � 2 � �, 0� � � � - ��� 9 � � � - #� ���#� {�Z)ikl T W�l > Zga lpT_T f�a �8Zgf�T�jVac^�j_}k��l[ioac^ acb � � r iol \]a?~�i�7 T_~�ikbH}oT_^ $ Zgf�l)~�a ^�a�Z W�~�~,�'S,T87 ^�TWh^�T8� W�lpl[azj_ikWYZgion`T�U�Xgaz~���jVZ�ac^�� Z[fXgac� $ f 3�& E � 35EBikb�}kTV^ $ Zgf�l�W�~�~�W�^�~ ,Q. &�,Q.��U,Q.Hy�b a�XW�}k}I*�+% �-� � f�ikl+ikl�|��lpZ+Zgf�T�q�T_j !`T�W�} $ T = XgW \���}oZ[ioU }kioj WYZgioac^h~�T87 ^�T_~MW�l+ik^�lgW > ���bJ$t���WYZ��#� we���

h�� � h

Page 15: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

� f�T_^ "��� " B � (" ^ B y$Zgf�TBj_a�X[XgT_l(U�ac^�~�ik^ $ ik^`ZgTVXgl[TVjVZgikac^�ikl lpZgik}o} U�Xga�U�TVX]W�^�~ Zgf�T�XgT_lpZpXgikjVZ[ikac^acb � � �Bf W�l $ TV^�TVXgikj 7 = XgTLacb+~�ik\ T_^�l[ikac^'JN� 3/� � JN�7E � � J�� 3 &�E ���edMa�XgT_avn`TVX ac^�TL\hW > ~�ioX[T_jVZg} >j�f�T_j !eZgf WYZ � �� � B � (� ^ B � q�a"�&TVn`TVX,Zgf�T�j_a�X[XgTVlpU�ac^�~�io^ $ ik^`ZgTVXglpT_jVZgioac^�iol�^�a�Z�U�Xga�U.TVX�f Wvnzik^ $W�~�ik\]T_^�lpikac^ � f�ioj�f ikl Zga?a $ XgT WYZ =?> JN� 3/� � J�� E�� � JN� 3�&�E�� �)f�ikj�fBikl W�}kl[a Zgf�T�~�ik\ T_^�l[ikac^Lacb Zgf�T$ T_^�TVX[ikj67 = X[TN�

� �+*,� L > � � ;]Zgf�T lpTVZ (" � (" B ikl�j_}kaclpT_~�W�^�~#i Zgl�U�X[T_j_iklpT b a�Xg\ j W�^ = T XgT_W�~#aN5�b Xgac\ ; � x��'&���y� ��� W�^�~ � � x �Lq�a"� T n`TVX Zgf�Th~�TVZ[TVXg\]io^ WYZgikac^#acb (� � (� B ikl WB\ ��j�f1\]a�XgTsl[� = Zg}oTS*z��T_l(Zgikac^1W�^�~ik^�~�TVT_~Bi Z Z�W!`T_l+W \���j�f�\ a�XgT�j_ac\ U }oT��hb a�Xg\<� 5�a�X+ac�X U �X[U.acl[T_l ioZ iol T_^�ac� $ fLZga�j_ac\ U �ZgT�Zgf�iolj_ac^`n`ac}k�Zgioac^ U�Xga?~���jVZ �)f�T_^?3 ��,/.Bb a�X l[ac\]T�*��% � � f�T�l[io\ U }ki37 j WYZgioac^sZgf WYZ&X[T_l[��}oZ[l&ikl$~���T ZgaZgf�T$bDW�jVZPZgf WYZ � T'j_W�^ U�XgT_j_iol[T_} > ~�TVZgT Xg\]ik^�T (� ��� �P{|^�~�T_TV~ � ���-����� ��� +� I� ,Q.5����� � ^�. ���V,Q.5����yl[io^�j_T<^ .siolZ� lpZ�W = }oTN� � f�T_^ (� � � � ^�.� �Z.+� 5 ik^ W�}k} > (� ���-� �, �� (� ��� yYl[ik^�j_T ��� � iolPj_ac\ U }oTVZgTN�{�Z b ac}o}ka � l ik^�UFWYX[Zgioj_��} WYX Zgf WYZ (� � �eiol)l[\ aza�Zgf,�

� ���#� ���U�U.acl[T ��� ��� & � � � f�T�b ac}o}ka � io^ $ j_ac^�l[io~�TVX�WYZgioac^ iol j_ac^�n`TV^�ikT_^`Z ik^1j_ac\ U �Zgio^ $ Zgf�Tik^`ZgTVX[l[T_jVZgioac^ � & �� � �]�G� +�� &��� � �]���P���,��ik^�j_T� ik~�TV^�Zgi37 T_l&� i Zgf � �� �Y& � ��y.Zgf�T io^�Z[TVXgl[T_j Zgikac^� &��� � � � � + � &��� � �'� �$io~�T_^�Z[i�7 T_l �)ioZ[f � �� � �� � �)f�ikj�f ikl+Zgf�T�j_ac}k}oT_jVZgioac^ acb�*?� W�~Xg�U }kTVl

- � 3 ��� � ��� � ������<�� ��% � �"L 3.� � c��Y��� � � +?� c��Y��� � � +F� c��Y��� �N� / >

5 i �+* � % W�^�~<�)X[ioZgT ,/.el[io\ U } > W�l ,Y��'�zI�I�C � �5( 9 9 ��� � � �.� �$� 2 � � �"�.� , � W/� # � ���. (� � � (� : � (� : #> Jv��� � � ��^�T�f W�l � &��� � � (� � �<� �F� ^ .� -/� / �D.� �M� �)f�ik}olpZ� &��� � � (� : � � �F� ��� �+ : I���� -/� / � - ��� / � � � {|^�Zgf�TBl[T_j_ac^�~ ZgTVX[\ �;\hW >�= T Z�W!`T_^�ac�Z]acbZgf�T j_}kaclp�XgT a�U.TVX�WYZgioac^,� � f�T_^ W�lBio^ � � r�\hWYZ[j f�io^ $ Z[f�T�l[TVj_ac^�~9W�^�~ Zgf�i Xg~ T_^�ZpXgikT_l � T j_ac^ )j_}o��~�T�Zgf WYZ�Zgf�TLXgT_lp��}oZgio^ $ io^�ZgT Xgl[T_jVZ[ikac^�iol Zgf�T �0lgWYZ[�X�WYZgikac^�l[TVZ acbG� ^ . !-/� / � .� �M� +_Z.�� � � �+ : I� �D. � -/� / � - ��� / ����� m�^�~�TVX Zgf�Tsf > U�a�Zgf�TVl[ikl�acb$Zgf�Ts}kT_\]\]W + : '&U^ .�� � f���lZgf�iol ik^`ZgTVXgl[TVjVZgikac^Bikl+|��lpZ+Zgf�T�Xgi $ f�Z+f W�^�~�bDW�jVZga�X io^��)f�ikj�fLZgf�T,l[T_j_ac^�~BT_^`Z[X > ikl&X[T_~���j_T_~�Zga -/� / �{�Zgl �;lgWYZ[�X�WYZgikac^Bl[TVZ)\hWYU l ��^�~�TVX � � � Zga (� : yFW�l+XgT�*?��ioXgT_~,�

h�� ` h

Page 16: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

� ���,� J TVZF��� ���"& � = T�j_}kaclpT_~�l[� = nYWYXgikTVZ[ikT_l�����TLl[f W�}o}+lgW > Z[f WYZg� �&���+iol�U�X[a�U�TVXsikb Zgf�Tik^`ZgTVX[l[T_jVZgioac^ � &��� � ���G� + � &��� � �]� � �)ikl,U�Xga�U�TVX�� {(b , � W�� y�Zgf�TV^ (�5. � (� : ikl �� 2 U�Xga�U.TVX:� {|^�~�T_T_~Zgf�T�j_a�X[XgT_l(U�ac^�~�ik^ $ ik^`ZgTVXgl[TVjVZgikac^ f W�l ~�ik\ T_^�l[ikac^ � f�ioj�f T���jVT_T_~�l)Z[f�T�nYW�}k��T,Zga = T�U�Xga�U.TVX =�> t��

> Jv������� G ��GD��N � �5( 9 9 � � � � � � �$� 2 � ���"�3� , � T@� # � ���. (� � � (� : � � 9 ��� 9 �.� # �������� �)�(� � �,(� : � �� '$( � * ��- �. �� � �� 0� �<; *,��- �) �� � �� ; O � ( 9 2 � 4�� -:9 �, ��. 2 ��<;43� *,��- �) �� � �� �� � (� : #> Jv��� � � f�l'ik^ � � ��\hWYZgj�f�ik^ $ Z[f�T�l[T_j_ac^�~�W�^�~hZgf�i Xg~hT_^`Z[XgikTVl'Zgf�T�j_a�XpXgT_lpU.ac^�~�ik^ $ ik^`ZgTVXglpT_jVZgioac^�iol|��lpZ�Zgf�T � lgWYZ[�X�WYZgikac^�l[T Z�acb � ^�.� -/� / � . ��M� + _Z. � ��� �+ : 5� �D.Z� -/� / � - ��� / �����P{|^�j W�}oj_� )} WYZ[ik^ $ Zgf�iol io^�Z[TVXgl[T_j Zgikac^�� T,\hW > i $ ^�a�X[T�Zgf�T�l[T_j_ac^�~ W�^�~�Z[f�ioXg~LT_^`Z[XgioT_l�� � f�T_^LZgf�ikl io^�Z[TVXgl[T_j Zgikac^ikl�|��lpZ+Zgf�T6_Z.MlgWYZg�XgWYZgikac^Ll[TVZ acb � ^ . � � + ��� �+ : � - ��� / � � 4�a"� ��� �+ : 5� - ��� / � � (� :}kioT_l+ik^ � io^��)f�ikj�fF^�. � f W�l+j_a?~�ik\]T_^�lpikac^et���m�^�~�TVX)Z[f�T�f > U.a�Zgf�T_l[iol+acb Zgf�T�U�Xga�U.acl[ioZ[ikac^ ;+ : �& ^�. W�^�~ lpa (� : �& ^ . �-� {�Zhb ac}o}ka � l =�> �,Xg��}o}�� lsZgf�T_a�XgT_\ ��W�l[l[a?j_i WYZ[T_~ U�Xgik\ T_lacb WBU�X[ik^�j_i UFW�}�io~�T W�}�f Wvn`Tsf�T_i $ f`Z WYZ \]aclpZ�ac^�T2��Z[f WYZ Zgf�iol ik^`ZgTVX[l[T_jVZgioac^#ikl�T�*z��io~�ik\]TV^�l[ikac^ W�}�acbj_a?~�ik\]T_^�lpikac^ ac^�TN�gJ TVZ � � � � � � > > > � ��� = TsioZgl,ioXpXgT_~���j_i = }kT jVac\ U�ac^�T_^`Zgl�� � f�T_l[T WYXgTsa = nioac��l[} >� lpZgW = }kT�W�^�~ f Wvn`T�~�ik\]TV^�l[ikac^B~�ik\ (� : � t ��O � t�����U�U.acl[TR_Z. � 1�� � 1 � � f�T_^ _Z. � 1 � f�ioj f ikl jV}kacl[T_~�yFioXpXgT_~���j_i = }kT�yF}kioT_l io^ � W�^�~ f W�l)~�ik\]TV^ )

l[ioac^+O y�\���lpZ = T lpac\]T (� �� � ��� y =�> ; �>=��M4�a � jVac^�l[ik~�TVX Zgf�T XgT_io^�l[TVX[Z[ikac^MacbPZgf�T l[T_j_ac^�~eW�^�~Zgf�i Xg~ i $ ^�a�XgT_~ TV^�Z[X[ikT_l�� � f�TsZgf�i Xg~ i $ ^�a�XgT_~ TV^�Z[X > �)ik}o} = T \]az~�i37 T_~ =�> Zgf�TsW�jVZgikac^�acb _Z.�2 = �ZZgf�iol)iol acbP^�a j_ac^�l[T�*?��T_^�j_T l[io^�j_T i Z iol)TV}kik\ ik^ WYZgT_~Bac^ WYU�U } > ik^ $ � � � �$�)T_ik^�lpTVX[Zgioac^ acbPZgf�T�l[T_j_ac^�~i $ ^�a�X[T_~�T_^`Z[X > �)f�ikj�f ikl -/� / W�^�~1Zgf�T ��l[WYZg�X�WYZgioac^ acb+Zgf�TLXgT_l[��} Z $ i n`T (� �eW $ W�io^ �U ZgaeZgf�TioXpXgT_}kT n�W�^`Z'Zgf�i Xg~]T_^`Z[X > � � f?��l$Zgf�ikl&jVac\ U�ac^�T_^`Z y�^ W�\]T_} > � 1 $ ion`T_l'W j_ac^`Z[Xgi = �Z[ikac^ (����Z[a (� ��� (� : �

5 io^ W�}k} > lp�U�U�acl[T&_ . � 1�� � 1 � � f�T_^�� 1 & (� : W�^�~]f W�l�j_az~�io\]T_^�l[ioac^ t�� � TVik^�l[TVXpZgikac^]acb�Zgf�TZgf�i Xg~hi $ ^�a�XgTV~]T_^`Z[X > W�^�~LWYU�U } > io^ $ _Z. �)ik}o}FXgT U } W�j_T�i Z =�> �D.�� � f�ikl'io^�jVXgT W�lpT_l&Zgf�T�~�io\]T_^�l[ioac^ ZgaO'� 5 ik^ W�}k} > XgT_io^�l[TVX[Z[ik^ $ Zgf�T�l[T_jVac^�~ i $ ^�a�XgTV~ T_^`Z[X > � f�ioj�f ikl#-/� / W�^�~ W�jVZgik^ $ =?> � �)ik}k}�b �XpZgf�TVXik^�j XgT W�l[T Zgf�T ~�io\]T_^�l[ioac^�Zga ; O'�Ff�U�U } > io^ $ � � � > ikT_}o~�l W�j_}kaclpT_~#ioXpXgT_~���j_i = }kT lp� = nYWYXgikT Z > acb (� : ���ik^�jVT � � � ikl � T8*z��ionYWYXgikW�^�Z ioZ�b ac}o}ka � l�W�l ik^ � � rsZ[f WYZ � � � f W�l $ T_^�TVX[ikjI7 = XgT�iklpac\]a�X[U f�ikj�Zga�� .�4j_ac\ ik^ $ b Xgac\ Z[f�T Zgf�ioXg~eT_^`Z[X > ��� � f���l�� 1 $ ion`T_l,W]j_ac^`Z[Xgi = �Zgikac^eZgaLW�^eioXpXgT_~���j_i = }kT�l[� = n�WYXgioTVZ >

h��04 h

Page 17: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

acb (� : acbPj_a?~�ik\]TV^�l[ikac^�t��� ���,� ��T�j W�^ \]W!`T � �C=s\]a�X[T,U�X[T_j_iklpT W�l b ac}o}ka � l8�

�'�zI�I�C � �5( 9 9 ��� � � � � �$� 2 � � � �3� , � T!� # � ���.� � ��� � � 2 �8� �3( 6 � � 2�� � : & - � � � �?W��� , �AW � / �3(543� 2 � � 2�'&�� (� � � (� : � (� � : ? ?

� ;����� �(���

( 9 2 � 4�� - 9 �� ��. 2 �� ;4�� *,�%- �. �� � �� �� � (� : #> Jv��� � � f�}oXgT_W�~ > (� � � (� : � � ��� � : � (� � : y =�> � ��� � � f�T)XgT_\]W�ik^�ik^ $ ZgTVXg\ l$io^`����Pj_ac\]T b X[ac\(� : � � : � f�ioj�fMf W�l j_a?~�ik\]T_^�lpikac^ �8t�ik^ (� : � 4�a"� (� : & % � &�� � (" : �&W�^�~ l[asZgf�ikl)j_ac\ U }kTV\]T_^`Z}kioT_l�io^ ?

��� :��� =�> ; � xzy��'&�� �.U'ac^�l[ik~�T X"H � (� : +,� � W�^�~el[�U�U�acl[TV, �?T �S� i Zgf�� �� �I� � f�T_^

(� ��� H � � ��� H�� H W�^�~ � � � H & � ������� �.� ���?y =�> � ��� �-f�}oX[T W�~ > ~�io\ H G ; O � t W�^�~� T]jV} W�ik\�Zgf WYZ ~�ik\ � � ��H G ; O � t���f�l ik^ � � � � T]\]W > \hWYZgj�f#�U�Z[f�T]l[T_j_ac^�~�W�^�~#Zgf�i Xg~T_^`Z[XgioT_l acb�� & �� � � (� �"� W�^�~ � &��� � �7HV�+Zga ~�T_~���jVTsZgf WYZ�Zgf�T_i X�io^�Z[TVXgl[T_j Zgikac^#iol |��l(Z�Zgf�T � lgWYZg�X�WYZ[ikac^l[TVZ�acb � ^�.� -/� / � . 4� � + _Z.�� ��� �+ : 5� �D. � -/� / � - ��� / � + ��� �+ � 5� �D. � -/� / � - � � / � ���f�l = T_b a�XgT-� T \hW > i $ ^�a�X[T)Zgf�T)l[T_j_ac^�~LW�^�~ Z[f�ioXg~]TV^�Z[X[ikT_l$ik^]j W�}oj_��} WYZ[ik^ $ Z[f�ikl'io^�Z[TVXgl[T_j Zgikac^��)f�ikj�fZgf�T_^ = T_j_ac\]TVlM_Z.�� � ^�. V�M�:+�� (� : + � � � ��yY�)f�ikj�fLikl �"l(Z�W = }kTN�&��io^�j_T�Zgf�T�W�jVZ[ikac^]acb �8ik^�jVXgT_W�l[T_l~�ik\ T_^�l[ikac^ =?> WYZ)\]aclpZ)~�ik\ ��� � ��O0��W�jVZ[� W�}k} > T � W�jVZg} >L=?> O#io^ nioT8�8acbPZgf�T�l[T_j_ac^�~Mi $ ^�a�XgT_~T_^`Z[X > �,ioZ ikl TV^�ac� $ f1Z[a l[f�a � Zgf WYZS� ^ .� �S� + (� : + ����y+�)f�ikj�f1ikl��0lpZgW = }kT�yHf W�l ~�ik\]T_^�lpikac^G O � ; � �'TVZ�, �MT �;W�^�~ , � T �hl[a = a�Zgf �+ : eW�^�~ )+ � MjVac^�Z�W�io^���35.+�GU'ac^�l[T�*z��TV^�Zg} >(� : +�� �.�)f�ikj�f]f W�l�~�ik\ T_^�l[ikac^ G O � t jVac^�Z�W�io^�l�� 35.BW�^�~]lpa ik^`ZgTVX[l[T_jVZgioac^g�)ioZgf�^ . ���b �XpZgf�TVX~Xga�U l�Zgf�T ~�ik\]T_^�lpikac^ =�> tcy�W�l�X[T�*z��i XgT_~,�){�Z b ac}k}ka �)l Zgf WYZ�l[��j�feZgTVXg\ l $ ion`T ^�ahj_ac^�ZpXgi = �Zgioac^ ZgaXgi $ f�Z)f W�^�~ l[io~�T acb � &����

5 io^ W�}k} > lp�U�U�acl[TVH ikl�W�j_}kacl[TV~ ioXpXgT_~���j_i = }kT jVac\ U�ac^�T_^`Z�acb (� : + (���S� i Zgf , � W ��� � f�T_^(� ��� H &�(� ����(����� (� �?y =�> � �>=�����ik^�jVT �?W � y�Xgi $ f�Z�f W�^�~ l[io~�T acb-�'&�� \hW > a = Z�W�ik^eW ZgT Xg\0acbZgf�T�b a�Xg\ (� �) � � � � : yFW�l XgT8*z��ioX[T_~,�� ����� � 4�a"� �&T j W�^ $ ion`T,Zgf�T j_a�XpXgT_lpU.ac^�~�ik^ $ XgTVl[��}oZ b a�X)Zgf�T � : � �.� �

h�� , h

Page 18: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

� RP�?�.Jv�I � �������� � �.�'��* � % �� �� � �,������ � ; �",Q. W/��� 2 ���) (�5. � (� : � (� : #������� � ; �",Q. T/��� 2 ���) (�5. � (� : � � 9 � � 9 �.� �, *

(� � � � (� : � (� : ��� ? ?� ;�� � � � � "%$

(� � "%$

( 9 2 � 4�� - 9 �� ��. 2 �� ;4�� *,�%- �. �� � �� �� � (� : #> Jv��� � � f�U�U } > ik^ $ J T_\]\hW � �>;��4ikioi �)Zga`� � (� � W�^�~ � � (� : � Thio\]\]T_~�ikWYZgT_} > ~�T_~���j_TLZgf�TXgT_lp��}oZ�b X[ac\ � � u��

� ����� � � f�T�l[TVZ � ��� : iol&bDWYX b Xgac\ T_\ U�Z > W�^�~hio^ $ T_^�TVX�W�}�~�i�� j_��}oZ&Z[a ~�TVZgTVXg\ ik^�TN�H��T l[f W�}k}�l[T_TZgf WYZ)ioZ ikl)~�TVZgTVX[\]ik^�T_~Bik\ U }oikj_i Zg} > =?> l[io^ $ ��} WYXgi Z > ~ WYZ�W�� jVac\ = io^�T x � ry�x � � W�^�~ex �kt"; �

�#� � ��N ���AP�2��G �.N GON�� �.Jv�A���� ���.Jv�#RP��I��.A � �F��#����� {|^X�)f WYZ,b ac}k}oa"�)l�Zgf�T�j_azT��]j_ioT_^�Z,Xgio^ $ ikl�W�l[l[��\]TV~eZ[a = T�� W�^�~ Z[a�U�ac}ka $ ioj W�}Pl(UFW�j_T_l�WYXgTW�l[l[��\ T_~�}oazj W�}o} > j_ac\ UFW�jVZ�� 5Fa�X,l[��j�f#WLlpUFW�j_T�}oTVZ6� �1 � HH� ~�T_^�a�ZgT�ioZgl.c�)4Zgf l[ik^ $ ��}kWYX,f�ac\]ac}ka $ >lpUFW�j_TLW�^�~ �H � -QH � / ioZgl�ac^�T�)�U.acik^`Z�j_ac\ UFW�jVZgi37 j WYZgioac^,� � f�TSc�)4Zgf�L a�XgT_}3) dMaza�XgTsf�ac\]ac}ka $ >lpUFW�j_T � 1 � HH�$acb H ikl ~�T87 ^�T_~ Zga = T�Zgf�T6c )�Zgf X[T_} WYZgi n`T�lpik^ $ ��} WYX)f�ac\]ac}oa $ > lpUFW�j_T_lVyFZgf WYZ)ikl

�1 � HH�< ��� �1 ���H �0� >�#�PO#� J TVZAH = T�WMj_ac\ U }kT �1W�} $ T = X�W�ikj]nYWYXgikT Z > acb jVac\ U }kT��#~�ik\]TV^�l[ikac^Td j_ac^�lpik~�TVXgTV~�ik^1i Zglj_ac\ U }oT��MZga�U.ac}ka $ > �&J TVZ)-QH � / � ;���� ~�T_^�a�ZgT Zgf�T�l[T Z�acb�ioXpXgT_~���j_i = }kT�j_ac\ U.ac^�T_^�Z[l�acb H f Wvnzik^ $j_ac\ U }oT�� ~�io\]T_^�l[ioac^\d � � f�T_ioX�b ��^�~ W�\ T_^�ZgW�}j_} W�lpl[T_l?� H�� � ��������hycb a�X[\ W = W�l[ikl b a�X � � � �7HV��yZgf WYZ)ikl

� � � �7HV� � �� ;����

� � H � ��>

h�� F h

Page 19: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

�#���,� � TVj W�}k}�Zgf WYZ Zgf�T��`ZgT_ik^ = T X $ n�WYX[ikTVZ > � f W�l jVac\ U }kT���~�io\]T_^�l[ioac^�; O y �)f�TVXgTO � L !$) L>�dMa�XgT_avn`TVX ��iol T�*?��ik~�io\]T_^�l[ioac^ W�}2W�^�~ ioZgl ioX[X[T_~���j_i = }oT j_ac\ U.ac^�T_^`Zgl WYX[T�Zgf�T (� : � �8� � � f�T_^=�> x �>;

�����+�'� ��� �:<;�=

��� (� : �7>

f�j_jVa�Xg~�ik^ $ Zga @�io^�� = �X $ �C@ y U @ y r����N�)Zgf�T W�} $ T = XgW ����� �'� � ik~�TV^�Zgi37 T_l � i Zgf Zgf�T $ Xgac�UW�} $ T = X�W �#� � 5Fa�X6� ��� � TG�)ik}o}2~�TV^�a�ZgT =?> Zgf�T lgW�\]T�}kTVZpZgTVX�Zgf�T jVa�X[XgT_lpU.ac^�~�ik^ $ T_}oT_\]T_^`Z�acb����� �'� ��� � f�T ��^�i Z acb �����+� � �$ikl $ ion`T_^ =?> Zgf�T�j_} W�lpl�t � � (� � �]�� f?��lVy�Zgf�T�n`T_jVZga�X lpUFW�j_T������ �'� � W�~�\ ioZgl Z�� a = W�l[TVl_yFZ[f�T ac^�T- �$L � �8� / y�W�^�~ Zgf�T a�Z[f�TVX

-�� (� : �8L � �8� / y W�^�~ l[asac^�T \hW > }ka?aN!�WYZ Z[f�T j_a?T8�]j_ioT_^`Zgl acbPZ[f�T�Z[XgW�^�l[ioZgioac^ \hWYZpXgi3�

� (� : � � �B�; =

�\����� E���E5>

� f�T > lgWYZgiol[b > �\� ��� �6� � t+W�^�~\�\����� E � � w���^�}kT_l[l�E?G/�I�+f�j_j_a�Xg~�io^ $ Z[a��CU-@?��y6} W�lpZPUFWYX�W $ X�WYU fik^ r���� �otvryZgf�T�jVazT8� j_ikT_^`Zgl)acbPZgf�T io^�n`T Xgl[T \]WYZ[Xgi3�LWYXgT�U.acl[ioZ[ion`T�ik^`ZgT $ TVXgl8�

�h�I�CFJ�� � ��W��_f�~ W�^ W�^�~KJ2��l[� Zgi $ � �6J+;2�+f W�~ U�X[TVnioac��l[} > $ i n`T_^ WMZga�U�ac}oa $ ikj W�}'j_ac^�lpZ[Xg��j Zgikac^acb�W;� 8� W�j Zgikac^Mac^ � ��� �'� �+\hW!io^ $ ioZ�ikl[ac\]a�XpU f�ikj Zga ��� �)ioZgf (��� j_a�XpXgT_lpU.ac^�~�ik^ $ ZgaF���� a a = Z�W�ik^`@�io^�� = �X $ � l)XgT_lp��}oZ)ikl+T_^�ac� $ f Zga l[f�a �9Zgf WYZ Zgf�ikl+}kTVb Z � W�jVZ[ikac^ jVac\]\��ZgT_l �)ioZgf�Zgf�TXgi $ f�Z \���}oZgi U }kioj WYZgikac^Lio^+� ��� �'� ���$�)acl[l[\hW�^�^T� �-��y��)f�acl[Tb�&a�X !hXgT_}kWYZgT_l+Zga = a�Zgf�� @?� W�^�~T���bJ�;2�4yj_}kWYXgi�7 T_~BZgf�T $ TVac\]TVZ[Xgioj,^ WYZg�XgT acb2Zgf�T�jVazT8� j_ikT_^`Zgl �\����� E�� � q�T�U�Xgavn`T_~ Z[f WYZ b a�X E?G/�

�\����� E � �k� � t� � : ! & � B ! �'� B � � : ��)f�TVXgT �$� B �'� : �&ikl+Zgf�T �'��}oTVX)^?��\ = T X�acbPZgf�T�U�acik^`ZR� E5��we� � (� : � l[T_T W�}ol[aX� S�� = y deW�j�y �.������{�Z�iolWsl(Z[Xgikj Zg} > U.acl[ioZgi n`T�io^�Z[T $ TVX)ikbZE?G/� yFW�^�~ T8*z� W�}�Zga�t ikb �7E5��we��iol W l[\]aza�Z[f�U.acik^`Z:�

4�a�ZgT,Zgf�T�b ac}k}ka �)ik^ $

�'�zI�I�C ���" �� � ��� � (� ��� � , � t � ; ����, �� � ��-:9 7� � � � �34 2 � �� ,�#h�� � h

Page 20: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

> Jv��� � � ��io^�j_T (� � � (� �;� (� �YyPZ[f�Thj_} W�l[l � (� ���$l[f�ac��}k~ = T ik~�T_\ U.a�ZgT_^`Z �U1ZgaMW jVac^�lpZ�W�^`Z:� � f�TU�Xga�U.TVX[ZgioT_l acb,�\����� E �Hio\ U } > Zgf WYZG� (� ��� ��, ���-�)f�TVXgT��)iol W ^�T $ WYZgi n`T io^�ZgT $ TVX W�^�~hac^�} > �$� � t$ ion`TVlG��, ��� � � U�Xga�U.a�X[Zgioac^ W�}�Zga?, ���2�� f�T }oT_\]\hW�W�}klpa b ac}k}oa"�)l b Xgac\ � acl[l[\]W�^�^ � l b a�X[\ ��}kW lpik^�j_T (� �,ikl l[\]a?a�Zgf,�� fXgac� $ f Zgf�T�ik^`n`ac}k�Zgioac^�� 3 � E � �� �7E � 3 �$ac^��% �� ac^�T�a = Z�W�ik^�l

�'&�� �\��� � E���� �\�7E &�� � � & � ��>

�#��� � f�l ^�a�ZgT_~ io^ Zgf�TBik^`Z[Xgaz~���j Zgikac^�y&b ac}o}ka � io^ $ ac�X Zgf�T_a�X > acb&_bacEd [R��Q�� jVac^�j_TVXg^�io^ $ ik^UFWYX[Zgioj_��} WYX$a�Xg~�TVX$XgT_}kWYZgikac^�l'W�^�~ @�ac}o~�ikT+X�W�^�!sU.ac} > ^�ac\]i W�}ol_ycioZ �+W�l$XgWYZgf�TVX&^ WYZ[�X�W�}�Zga j_ac^|TVjVZg�XgTZgf WYZ�Zgf�TeU�XgT_l[T_^�jVT#acb�Z�� a l[��j�f = W�l[T_l b a�X-��� lpf�ac��}k~ }oT W�~ Zga l[ik\ ik} WYXhX[T_l[��}oZ[l b a�XBa�X = ioZ�W�}nYWYXgikTVZ[ikT_l�� � f?��lMio^ �1 ;zy ��T_jVZ:� ;2�6�&T#~�T_l[jVX[i = TV~ 7pj�f WYX�W�j ZgTVXgiolpZgikj�U�ac} > ^�ac\]i W�}ol 9 b a�X a�X = ioZ�W�}nYWYXgikTVZ[ikT_l�W�^�~MjVac^|T_jVZg�XgTV~ �1 ;zy.u��>=� Zgf WYZ Zgf�TVioX�U�XgT_jVikl[T b a�Xg\ l[f�ac��}k~ = T $ ion`T_^Mio^ ZgTVXg\]l)acb Zgf�T= W�l[iolI�\���6�� � �

B�;�=�\����� E���EMacb��#� � � f�ikl jVac^|T_jVZg�XgTS� W�lG7�XglpZ TVlpZ�W = }kiol[f�T_~ =�> � aclpl[\hW�^�^

� ��y���T_jVZ:� tct��E��S,i�5�T XgT_^`Z U�Xga?acb l-� T XgT $ i n`T_^ ik^T�1 �x2� W�^�~ io^�� %&�]�

�#���#� � f�TBj_ac^|T_jVZg�X[T��1 ;zy'u��>=�b���)f�ikj�f�iols^�a � Zgf�TLZgf�T_a�XgTV\ = T_}ka �b� b a�X Zgf�TBa�X[~�TVXgik^ $ � =�>ik^�jV}k��l[ioac^#��acb�a�X = ioZgW�}�nYWYXgikT Z > j_}oacl[�XgT_l j W�^ = T&T��zU�XgT_l[l[TV~sW�lHW�~�T_l[jVXgi U�Zgikac^ acbZ[f�T'Xgi $ f`Z $ T_ac\ TVZ[Xgiojj_ac^�T_l8� � f�ikl $ a?T_l W�l b ac}o}ka � l8�`@�ion`T_^1W�^ > lp� = l[TVZ �[& �����+�'� ��y2}kTVZ<� UA�$~�T_^�a�ZgTsZgf�T]l[\hW�}o}kT_l(Zl[� = l[T Z�acb�� l[��j�f Zgf WYZ

�Y& �:<;�� ��

� � (� : �7>

{�Z iol�l[ac\ TVZgik\ T_l�jVac^�n`T_^�ioT_^`Z ZgaBnzikT8� � UA�'W�l�Z[f�TsXgi $ f�Z�f W�^�~�l[ik~�T]W = avn`TN� � �X \hW�io^�XgT_lp��}oZ iolZgf�T�b ac}k}ka �)ik^ $

� RP�?�.Jv�I � � "a��%� �.�E �K��� (� : � � � �� E � �F���6��>

�#�+*,� f�l]ik^ Zgf�T j W�l[T acb�Zgf�TMa�X[~�TVXgik^ $ acb�U�X[ik\]i Zgion`T�ik~�T W�}ol_y&Zgf�T io\ U }kioj WYZgioac^ � � ikls\ ��j�fT W�l[ioTVX y�Zgf�ac� $ f � T�~�ik~ ^�a�Z,XgT W�}oik�_T Zgf�iol,WYZ�Zgf�T Zgik\]T�,{|^�~�T_TV~#W�^ ik^`ZgT_}k}oi $ T_^`Z�XgT W�~�io^ $ acb&���bJ+;zy

h i h

Page 21: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

��T_jVZ�����H��l[io^ $ ik^ UFWYX[Zgioj_��} WYX Zgf�T l[f�a�X[Z,T���W�jVZ�l[T8*z��T_^�j_T�acb � �6J+;zy'� ; �>; � � $ i n`T_l Zgf�T io\ U }kioj WYZgioac^E �K� � � (� : � � � � � E � ���g� �6��� � T U } W�j_io^ $ Zgf�T��`ZgT_io^ = TVX $ n�WYXgioTVZ > � =�> Zgf�T)jVac^�a�Xg\hW�}znYWYXgikT Z >� $ i n`T_l =�> W lpik\]io} WYX)W�^ W�} > l[ikl)��l[ik^ $ Zgf�T�X�WYZgf�TVX \]T_l[l > ~�T_b a�Xg\]WYZgikac^�XgTVZ[X�W�j Zgikac^MWYX $ ��\]T_^`Zgl�acb��W��_f�~ W�^#W�^�~ J2��l[� Zgi $ Zgf�T�ik\ U }oikj WYZ[ikac^ E8� � � (� : � � � � � E.� �g� �6� ����Tslpf W�}k} ^�a�Z $ ion`T Zgf�iolik^B~�TVZ�W�io}2W�l � T�a = Z�W�ik^ Wsl[T_jVac^�~ U�Xgazacb = T_}oa"�6�

� f�T a�U�U.acl[ioZgT�ik\sU }kikj_WYZgikac^Lik^�Zgf�T,U�Xgik\]i Zgion`T�io~�T W�}�l[TVZ )��U �+W�l a = Z�W�ik^�TV~ =�> %$a $ W�^H� %$aN��ik^f�ikl,io\]\]T_~�ikWYZgT XgT_l(U�ac^�l[T ZgaBac�X�j_ac^|TVjVZg�XgTN� q�TR7�Xgl(Z ^�a�ZgTV~�Zgf WYZ,iob��4&�� iol�W�^ ik^�jV}k��l[ioac^ acb

U�Xgio\]ioZgi n`T'io~�T W�}kl acb [\� Q���y�Zgf�T_^�ac^�T f W�l�W�l[�X]|T_j Zgikac^ acb,[\��Q � ��� � �0[\� Q�� ���hacb *z��a�ZgioT_^�Z W�} $ T = X�W�l8�� b j_ac�Xgl[T]Zgf�T XgT]ikl W j_a�X[X[T_lpU�ac^�~�io^ $ l[�X]|T_jVZ[ikac^1acb'Zgf�ThW�} $ T = X�W�l�acb&XgT $ ��} WYX b ��^�jVZgioac^�l ac^#Zgf�Ta�X = i Z�W�} nYWYXgioTVZ > j_}oacl[�XgT_l��&f+Z?7�Xgl(Z�Zgf�ikl�WYU�U�X[a`W�j f�WYU�U�T_WYXgl�b Xg��ioZ[}kT_l[l�io^ Zgf�T $ T_ac\]T Z[Xgikj�j_ac^`ZgT��Z��{|^�~�T_T_~ ��^�}ki3!`T Zgf�T U�Xgik\ ioZgi n`T ik~�T W�}2j W�l[T Zgf�T_lpT�W�} $ T = X�W�l,WYX[T ^�a�Z.f�X[Zgik^�ikW�^,��dMa�XgT_avn`TVX,ac^�T�j W�^ik^`ZgTVXpU�XgTVZ-%'a $ W�^ � l l[T_j_ac^�~ l(ZgTVU W�l Zgf�T�XgT = ��ik}k~�io^ $ acb [\��Q�� ���sb Xgac\ ioZgl+l[a?j_}kT,��l[io^ $ Z[f�T �$^Xgi $ f`Zb ��^�jVZga�X��1 ;zy2x � � �]� � f�ikl�ioZgl[TV}kb�f W�l�^�aBa = nzikac��l�W�^ W�}ka $ ��T�ik^ Zgf�T $ T_ac\]TVZ[X[ikj�j_ac^`ZgT��Z�� q�a"� T n`TVXZgf�T]b ��^�j Zga�Xgi W�}�U�XgazjVT_l[l ioZ[l[T_}kb j_W�^ = Thik^`ZgTVXpU�XgTVZgT_~�W�l jVac^�n`ac}o�Zgikac^#io^TL a�XgT_}3) dMaza�XgT]f�ac\ ac}ka $ > �� f�T_^ Z[f�T\!`T > a = l[TVXpn�WYZgioac^X� W�l�Zgf WYZ�Zgf�T W�^ W�}ka $ ��T acb = ��ik}o~�ik^ $ �U�b Xgac\0Zgf�T l[a?j_}kT�iklA|��l(Z r�� �j_ac\ = ik^�TV~<� i Zgf � � ; ����T�lpf W�}k}�l[T_T�f�a"�8Zgf�iol-� a�X !l ac�Z = T_}ka �G�

�#���#� J TVZM� & � � �Y&�� = T�j_}kaclpT_~hl[� = n�WYXgioTVZgioT_l&acb�Zga�UL~�io\]T_^�l[ioac^,� � f�T_^�W�l$ik^��CU-@ y � ; � � � ue� �Zgf�TVX[T ikl)W = io}kio^�T WYX \]WYU

����� �]�G� +� � � � �R� � � � � �+�]� � �R��>� f�ikl \hW!`T_l�� � � �!�g� W }kT_b Z)j_ac^`n`ac}k�Zgioac^ \]az~���}oT,b a�X)Zgf�T�j_ac^`n`ac}k�Zgioac^ W�} $ T = X�W$����� �'� ����#���,� 5�ac}o}ka � io^ $ r�� ��W�^�~ � � ;V� TX!z^�a"��Z[f WYZ�Z[f�T�j_ac\sU�ac^�T_^`Zgl�acb � WYX[T XgTV}kTVnYW�^�Z�Zga�Zgf�T~�T_l[j XgioU�Zgioac^ acbPio^�j_}k��lpikac^ XgTV} WYZgioac^ acbPa�X = ioZ n�WYXgioTVZ > j_}kacl[�X[T_l��

� f�T b ac}k}oa"�)ik^ $ a = lpTVX[nYWYZgikac^eW�}o}ka � l)ac^�T ZgahW n`acik~ Zgf�T WYU�U.T WYXgT_^�jVT acb����!�g�+ik^ Zgf�T b a�Xg\�� )} WYZ[ikac^�acbPZgf�T�\hW�ik^LXgT_l[��} Z,x � x �

�'�zI�I�C � � ��� - � 9 ��(� : � �� � (� :#"&$ � � � 2 �) * � � � �,�. � 2 � �, � � � - �,� 9 � � � - � ��� �'� � �?� � � ���\��<;�7� ; 2 � - � * (�7�.� #h i��Bh

Page 22: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

> Jv��� � � � f�iol ikl WMb a�Xg\hW�}'jVac^�l[T�*z��TV^�j_TBacb+Zgf�T = W�l[TLj�f W�^ $ TLb a�XsjVac^�n`ac}o�Zgikac^�U�Xga?~���jVZ y�l[T_Tf�U�U�TV^�~�i3��y fb; ��{|^ bDW�jVZ y�Zgf�T�T_\ = T_~�~�io^ $ � �-/� / � � ik^�~���jVT_l�W \hWYU

�� �� � � �� �� �lgWYZgiol[b > io^ $ Z[f�T�jVac^�~�ioZgioac^�l2acb 3$Xga�U.acl[ioZ[ikac^.fb; � r�� � f�TVX[T_b a�XgT�yYW f�ac\]ac\ a�X[U f�ikl[\ � ��� � � � �?� � � ���F�acbI� ��� �'� �2\]az~���}oT_l�ikl�ik^�~���j_TV~,��{�Z�XgT_\hW�io^�l Zga�j�f�T_j ! Zgf WYZ�ioZ$j WYX[XgioT_l�Zgf�T+j_} W�l[l&� (� : ��Zga�Zgf�T j_}kW�l[l� (� :#"%$ �E� J2T_\ \hWFfb; �>; �OZgf�T = W�lpT j�f W�^ $ T b a�X?L d f�ac\]ac}ka $ > �$WYU�U }kikTV~ Zga�Zgf�T j WYXpZgT_l[ikW�^ l *?� WYXgT

� : "&$ �+�

� :�

� � �

ik\sU }kikTVl'Zgf WYZ&Zgf�T�io\hW $ T�acb � (� : ��ikl+W \���} ZgioU }oT�acb � (� : "&$ �]� � f�T,j_azT��]j_ioT_^�Z+iol ac^�T�l[ik^�jVT,Z[f�T�\hWYU� : "&$ � � : }ka?aN!l+}ka?j W�}k} > W�l Zgf�T��_TVX[a]l[T_jVZ[ikac^ acbPWsj_ac\ U }oT��Ln`T_jVZ[a�X = ��^�~�}kT��#���,� ��T�\hW > ^�a"� $ i n`T W l[T_j_ac^�~ U�X[azacbHacb Zgf�T io\ U }kioj WYZgikac^�� � acb Zgf�T_a�XgTV\0x � x ��{|^�~�T_TV~

�#�j� (� : � �������+� � ��� (� : ��� =�> x�> r& � � � � � � (� : � � =�> x�> ���� � � � % &�� % � (� : � ��� =�> � >>;��]cEc �� �B�; � : ! � (� B ��� =?> r > � �]c(� W�^�~ r > � >

��ik^�jVT����j� (� : � & � � � (� : ����y�Zgf�T,XgT�*?��ioXgT_~ W�l[l[TVX[Z[ikac^ b ac}o}ka � l b Xgac\ x �>=sW�^�~Mx � r � �����#����� � � a a = ZgW�ik^ Zgf�T ik\ U }oikj WYZgioac^ ��� acb'ac�X Zgf�T_a�X[T_\ i ZG� ac��}o~ = T]TV^�ac� $ f#Zga l[f�a �;Zgf WYZ� ��� �'� ��� (� : ��� � � � � � � (� : � � ��b�j_ac�X[l[T Zgf�iol iklsZgaza�\ ��j�f Zga�T��U.T_jVZLW�^�~ io^�~�T_T_~ TVn`TV^ ik^Zgf�T = TVZpZgTVX)��^�~�TVXglpZga?az~�U�Xgik\]i Zgion`T�io~�T W�}�l[TVZ ) �U Zgf�T�T��iklpZgTV^�j_T acb T_^�ac� $ f j > j_}kioj,T_}kT_\ T_^�Z[l)acb���4^ W�\]TV} > Zgf�acl[TLlgWYZgiol[b > io^ $ ��� � (� : � � � � (� : � ��ikl ��^U�Xgavn`T_^,�K� � f�Th}oac^ $ T_lpZ�T_}oT_\]T_^`Z�ik^1T W�j�f��T > }�l[� = $ Xgac�Ueikl)j > jV}kikj 2 = �Z)Zgf�T_l[T bDW�ik}�Zga T ��f W���lpZ�W�}k}�Zgf�T jVac^�T_l��3�Ff�} XgT W�~ > ioZ)ikl)W ~�T_T UebDW�jVZ

h i i h

Page 23: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

�1 �N��Z[f WYZ Z[f�T�S,��Z asik^`n`ac}k�Zgioac^�l+U�X[a�nzik~�T,j > jV}kikj,T_}oT_\]T_^`Zgl+ik^LZgf�T,}kTVb Z j_TV}k}klb� Z[f WYZ+ikl Zga lgW > W�b Z[TVXac^�T f W�l bDW�jVZga�X[T_~ ac�Z Zgf�T j_ac^�TVl j_a�X[X[T_lpU�ac^�~�io^ $ Zga }ka � TVX�~�io\]T_^�l[ioac^#���

� ^ Z[f�T a�Z[f�TVX�f W�^�~si Z�ikl ^ WYZg�XgW�}?Zga�W�l ! iob�Zgf�T+ik^�j_}o��l[ikac^ �N� ��� �'� ��� (� : � � & � � � � � � (� : �ikl�W�^T�*?� W�}kioZ > � � f�iol �&ac��}k~ b ac}o}ka � iob�� T+j_ac��}o~slpf�a"� Zgf WYZ?��� (� B �(� (� : � � ��� � � � (� B � (� : �����$E �.� � � f�iolW�\]ac��^`ZglPZga�lpf�a"�)ik^ $ Zgf WYZPZgf�T b ��^�~ W�\]T_^`Z�W�}zjV} W�l[l acb�TVn`T X > i X[XgT_~���j_i = }kT j_ac\sU�ac^�T_^`Z�acb (� B � (� : acbj_ac\ U }oT��h~�io\]T_^�l[ioac^?OMazj_jV�XglM� i Zgfh^�ac^ ) �_T XgasjVazT8� j_ikT_^`Z+ik^]Z[f�T�j_ac^�n`ac}o�Zgikac^hU�Xga?~���jVZI� (� B �(� (� : �E�L > W�Zgf�TVa�XgT_\ acb)��TVX[XgTsioZ.� ac��}o~ = TsT_^�ac� $ f Zga�l[f�a � Z[f WYZ (� B (� : ikl,U�Xga�U�TVX ik^ Zgf�T l[TV^�l[T]acb� �>=y�l[TVTSf�U�U�T_^�~�i � f t���� b&j_ac�XglpTg� Tsf Wvn`T W�}oXgT W�~ > l[TVT_^ Zgf WYZ�U�Xga�U�T Xg^�T_l[l bDW�io}kl�W�^�~#l[aB\hW >ioZ[l W�^�Z[ikj_i UFWYZgT_~1j_ac^�j_}o��l[ikac^ 2 = �ZR� Thl[f�a � Zgf WYZ Zgf�T]X[T�*z��i XgT_~1j_ac^�j_}o��l[ikac^�ikl lp���]j_ioT_^�Z[} > j_}oacl[TZga = T_ik^ $ ZpXg��TT�4l[T_T 3$Xga�U.acl[ioZgioac^ x �kt � � =�> W�^ W�} > �_ik^ $ Z[f�T U�Xgaz~���j ZglS� f�TV^ E iolhW $ T_^�TVX�WYZ[ik^ $XgT8Z TVjVZgikac^,� � f�iol-�)ik}o}�U�Xgavn`T�Zgf�T�Zgf�T_a�XgT_\<�

�#����� � J TVZ ��lG7�Xgl(Z j_ac^�l[io~�TVX Zgf�T7 = W�~ 9 j W�l[Tg� f�T_^ E �a,/. * � % W�^�~%�",/. W �I�Bq�T XgT(� ��� ��(� : � (� : � 3�Xga�U�TVX[^�T_l[lG� ac��}o~�ik\ U } > Zgf WYZ � (� ��� �(� (� : �$ikl WBlpZ[XgiojVZg} > U.acl[ioZgi n`T \���}oZ[ioU }kT acb� (� : �]�$� WYZgf�TVX � T�f W n`T�Z[f�T�'�zI�I�C � �5( 9 9 ��� � *%�+%��6�,� � � �$� 2 � �%; � �",/. W/� # � ���)

� (� ��� �(� (� : � � � ;�� (� : ��>> Jv��� � � L > � � �I�&T�\���lpZ)f Wvn`T

� (� ��� �(� (� : "%$ � � �Y� (� : "&$ �b a�X l[ac\]T ��� �F� �'TVZ � (� � � � � � ��,/. � t� � � � ;���,Q. � t� � � ;�� (� � ����y =�> x � r�� � f�iol b a�X[j_T_l� � � ; � � f�T_^ex �>= $ ion`T_l+Zgf�T,XgT�*?��ioXgT_~ W�l[l[TVX[Z[ikac^,��#��� O � � T_j_W�}k}�Zgf�T ^�a�ZgWYZgikac^Bacb � �ktvw��> Jv������� G ��GD��N � �5( 9 9 � � � *%�+% �, * � �.� �$� 2 � ��; � ��,/.4T/� # � ���.

� (� ��� � � (� : � � � (� : ��� ��� �� ;�� � � � � "&$

� � � (� � "&$ ���

h i�? h

Page 24: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

1 ���)� � 2 ��� � � �,� � � 2 � � 4 2 � 9 � � ��2 � � � � 2 �����.��� #> Jv��� � � � f�T b a�Xg\���} W1b ac}k}ka �)lBb Xgac\ U�Xga�U.TVXg^�T_lpl ik^ � �ktvw �4ioi �sW�^�~BJ2T_\ \hWHf t�� ;�b Xgac\ Zgf�TWYU�U�TV^�~�i3�,� � f�T j_a?T8�]j_ioT_^`Z�acb � (� : � � � ikl ac^�T =�> ni X[Zg��T�acb � ���sW�^�~ f t��ktY��x ���

�h�I�CFJ � � � ^�T \hW > ~�T87 ^�T�Wsl[T_j_ac^�~ W�jVZgioac^ acb������ �'� � ac^4� � � ���g� lpa]Zgf WYZ",Q.�j_ac\ T_lac�Z�ac^ Zgf�T }kT_b Z�io^ Zgf�T b a�Xg\���}kW ~�T_l[jVX[i = ik^ $ Zgf�T�W�j Zgikac^ acb � (� ��� �Pac^ � (� : �E� � f�ikl�W�jVZ[ikac^Mj�f W�^ $ T_lZgf�T XgT_l[��} Zgik^ $ a�X = ioZ�W�}HnYWYXgikT ZgikT_l W�^�~#ikl�T_l[l[TV^�ZgikW�}k} > �)f WYZ a?j_j_�Xgl�ik^B� �6J+;zy ��T_jVZ:�B� �4y �� `ry���T_jVZ���N��y+� q y � f�\ �+t��4y+�1L L+d1y � �ot �N�E�

� f�T W = a�n`T l[i Zg� WYZgikac^ ikl�Zgf�T�U�XgT_jVikl[T W�^ W�}ka $ ��T�acb$Zgf�T T_^`n`T_}ka�U io^ $ W�} $ T = X�WLac^�TN��q�TVXgT�Zgf�Tj_acf�TVX[T_^�Z)j_ac^`Zgik^�� WYZgioac^ b ��^�jVZga�X[l $ i n`T�W Xgi $ f�Z)W�jVZgioac^Lacb � ac^LZgf�T#( j WYZgT $ a�X > 2 = �Z)ac^�T.�)f�ikj�fj�f W�^ $ T_l&W�^�^�iof�ik}kWYZga�Xgl��+%,ikW�Zgf�T)T8*z��ionYW�}kTV^�j_T)acbFj WYZgT $ a�XgikT_l�ik^`�1L @ �Zgf�T > WYXgT Z[XgW�^�l[b a�Xg\]T_~ Zga,Zgf�T�'^X[i $ f`Z�b ��^�jVZga�Xgl?�)f�ikj�fe~�ah^�a�Z,W�} ZgTVX,W�^�^�iof�ik} WYZ[a�Xgl�W�^�~eik\sU }kT_\]TV^�Z W]}kT_b Z�W�jVZgikac^MacbHZ[f�T ��T > }$ Xgac�U �1 `r�E� � f WYZ Zgf�T+Xgac}kTVl�acbF}kTVb Z'W�^�~ Xgi $ f`Z$WYXgT+ik^`ZgTVXgj�f W�^ $ T_~ ac^ UFW�l[lpik^ $ b Xgac\ XgTVU�XgT_lpT_^�ZgWYZgikac^Zgf�T_a�X > Z[a $ T_ac\]T Z[X > iol ^ WYZg�X�W�}�b Xgac\ �� e;zy���T_j Z:�&x2�2W�^�~ b X[ac\ ���>;��'&�����#��� �/� ��T lpT_T b Xgac\ x �ktctcy+x �ot";eZgf WYZsZgf�T j_ac^`n`ac}k�Z[ikac^�U�Xga?~���jVZ =�> � (� ��� � XgT_j_avn`TVXgl]W�}k}+Zgf�Tj_ac\ U.ac^�T_^`Zgl)acb2Zgf�T l[T Z)Zgf�TVa�XgTVZgioj�j_ac^`n`ac}k�Z[ikac^ T��j_TVU�Z Zgf�acl[T�acbPj_a?~�ik\]T_^�lpikac^ t��

q�a � TVn`TVX =�> � �ktvw �4ioiki � ik^BW l[TV^�l[T�Zgf�TVl[T WYX[T�TV}kik\ ik^ WYZgT_~ �)f�T_^ �&T�XgT U } W�j_T � (� ��� � =?> � (� � � � �� (� � ��y �)f�ikj�f =�> x �C= XgT U�XgT_l[T_^`Zgl",Q.+� � f�iol-�)ik}o} = T�\hW�~�T�U�X[T_j_iklpT = TV}ka �G���T X[T_\hWYX !�Zgf WYZ?�)f�TVXgT W�l�Z[f�T = T_f W nioac�X�acb Zgf�T }ka � TVX�~�io\]T_^�l[ioac^ W�}Pj_ac\sU�ac^�T_^`Zgl�io^ � �otvw

ikl jVXg��jVi W�}�Zga ac�X � a�X !.y�ioZ-�+W�l)acbP^�a j_ac^�j_T Xg^ Zga�Zgf�T W��Zgf�a�X[l)XgT_b T X[XgT_~ Z[a ik^B� T_\]WYX ! x �kt"; �

> Jv��� �s� � � � � � � f�T]ik\ U }oikj WYZ[ikac^� � io^�x � x �+W�l W�}oXgT W�~ > U�Xgavn`T_^1ik^�x � u�� � f�T U�Xga?acb acb Zgf�Ta�Zgf�TVX)ik\sU }kikj_WYZgikac^�ikl $ ion`T_^Bik^ x �ktvr��x �kt � �

�)T_j W�}k}Pl[ac\]T ^�a�Zgioac^�l�b Xgac\ �1 �t��E�&J TVZ�� = T WS7 ^�ioZgT�~�io\]T_^�l[ioac^ W�}2n`T_jVZ[a�X,lpUFW�j_T a�n`TVX�� a�X�;f Wvnzik^ $ WF7���TV~ = W�l[iol -� � � > > > �� � / ��B� ��NHG ��G �.N �

� � ��� �)7� - �. 2 � ��� � �4 ���7� * U�acl[i Zgion`T � 4��� �� � � � ���� ��� � � ; ����� 1�� 1 1 ��2 � 1�� w #h i � h

Page 25: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

� 6 � � � ��4 2 �����3( 6 � 9 � 4�� [U& � � �4�� �7� * 9 � � ��2 � � � � ; ��2 � � � �. ��.� � 2 � * 6 � 2 ����� � 2 [ ����� �<; ��2 �9 � � ��2 � � ��)7� - �) 2 � #5Fa�X�W�U�acl[i Zgion`T,n`T_jVZ[a�X � � �<� ��� � 1 � 1 ��ac^�T�~�T87 ^�T_l�i Z l[�U�U.a�X[Z�W�l

���U�U ��� ��� - c L� 1-T w / >� ^�T f W�l jV}kT WYXg} > ���U�U�� � � � � � ���9���U�U�� � � � �B���U�U�� � � ���<5F�X[Zgf�T Xg\]a�XgT�y�ac^�T�~�T87 ^�T_l

���U�UV[ � ?� ;���� �

���U�U ��� ��>

{�Z iol)jV}kT WYX)b a�X�W�^ > U�acl[i Zgion`T\[ Z[f�TVXgT T ��ikl(Zgl3���[�����lp��j fBZgf WYZ����U�UH[ �8���U�U � 3/���

��T WYXgT io^�Z[TVXgT_lpZ[T_~ io^eZ[f�T�n`TVjVZga�X,lpUFW�j_TA� � � ���F�>� " : ;�= ��� (� : ���)ioZgfMZgf�T = W�l[iol�-�� (� : � / �� TVj W�}k}�Zgf WYZ � � �+�!�F�&iol WV�����+� � �'\]az~���}oTN� ��T.7�� � �.� �

5Fa�X T_W�j f�E �.� ~�T87 ^�T� �7E � � � CDB � � (� � � (� : ��>�'�zI�I�C � � �7E�� � � � 9 ��� ��2 � � � �3( 6 � 9 ��4���<; � � � �!�g� ; �,� � � E � � #

> Jv��� � � {|^�~���jVZgioac^#ac^ JN�7E���� � f�T j W�lpT$E�� tsikl�a = nzikac��l��h���U�U�acl[T ��� E��)ikl,U�acl[i Zgion`TsW�^�~#}oTVZE � ��, E?T ELb a�X�W l[ik\sU }kT,XgT8Z T_jVZ[ikac^4,�� � f�T_^

�S�7E � � � � C � B � � (� e�(� (� : � � � CDB � � (� e�(� (� : ��� �34G E3+G(, 3

� � (� � e�(� (� : �7>

L �Z � � (� � � � ��� (� � �(� (� �2\]az~���}oa � � �� ��� (��� �Hb a�X�3�lgWYZgiol[b > io^ $ , 3 T�3 �.� ^MZgf�T a�Zgf�T X�f W�^�~�yikbZ, 34W 32y ��� (� e�(� (� : � & �S�7E�� � � f���l_y

�S� E��<& �S�7E � �_& �S�7E�� � � (� � � �S�7E ��>��ik^�jVT ac^BZ[f�T a�Zgf�TVX)f W�^�~ �S�7E � �'iklR� (� ��� ik^`nYWYXgi W�^`Z y � T�f Wvn`T

� �7E � ��� � �7E�� � � (� ��� �S� E���>

h i ` h

Page 26: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

� f�ikl�} WYZ[Z[TVX ikl U�acl[i Zgion`Thlpik^�j_TT� (� � �+j W�^ = TLT�*?� W�}k} > l[� = lpZgi Zg�ZgT_~ =�> � (� ��� � ; � f�ioj�f U�XgT_lpTVX[n`T_lU�aclpioZgi nioZ > �

�#��� � � 5Fa�X W�^ > j_}kaclpT_~ lp� = nYWYXgikT Z > � �&acb ��}oTVZSU&ac\sU/��� ����~�TV^�a�ZgTLioZgl lpTVZ acb jVac\ U�ac^�T_^`Zgl�acb~�ik\ T_^�l[ikac^ O.� ~�io\ �S�> Jv������� G ��GD��N � � ( 9 9 � � � (����� U'ac\ U/� (� B � (� : �)# � ���. � (���"� �����U�U �S� E��$#> Jv��� � � {|^�~���jVZgioac^ ac^ JN�7E ��� � f�T�j W�l[T�E�� t ikl j_}oT WYX:� 4�a"�9XgT U } W�j_T6E =?> E � ��, E?T ES�)f�TVXgT�,ikl)Wslpik\ U }oT�XgT�Z T_jVZgioac^,� � f�T_^

(�����XU&ac\ U,� (� B�� � (� : �_& U'ac\ U/� (� � �,(� B � (� : ��>� fXgT_T j_W�l[T_l j_W�^ azj_jV�X:�

�pt� (���.� U&ac\sU/� (� B � (� : � � i Zgf , � W ����{|^#Zgf�iol j W�l[TsZgf�T XgT_lp��}oZ b ac}o}ka � l b X[ac\ x �ktct W�^�~�Zgf�Tik^�~���j Zgikac^ f > U.a�Zgf�T_l[iol��

� ; � (� � � U&ac\sU/� (� B � (� : ��y (� � � (� � ikl+U�Xga�U�T X yFW�^�~ � &�� ��- , 1 &�� / � � � � "&$ ���re� � ikl)Wsj_ac\ U.ac^�T_^`Z acb W l[\hW�}o}kTVX ~�ik\ T_^�l[ikac^Bik^ (� B � (� : W�^�~ (��� iol�W j_ac\ U�ac^�TV^�Z)acb (� � � �\�{|^�Zgf�T�l[T_jVac^�~ j W�l[T j�f�a?acl[T � � �S�7E��

��� �l[��j�f Zgf WYZh���U�U �S�7E ��� ���U�U+��� � ��J TVZ � �

� � 1 � (� ��� �/� i Zgf � 1 T w ��1 � � 1�� � f�T_^� � (� � ����re� ��� � � 1 � � (� ��� � re�8� (� ��� �7>

� f�T Xgi $ f�Z�f W�^�~#lpik~�T acb$Zgf�T T�*z� W�}oioZ > j_ac^`Z�W�io^�l�� (��� � �)ioZ[f WLlpZ[XgiojVZg} > U�acl[i Zgion`T�j_a?T8�]j_ioT_^`Z l[io^�j_T� (� ���(� (� � ��j_ac^�ZgW�ik^�l i Z�W�^�~ ^�a j W�^�j_TV}k} WYZ[ikac^�l)WYXgT�U.acl[l[i = }oTN� � f�iol)io\ U }kioT_l Z[f WYZ

� (��� � �����U�U�� � � � ���U�U � � (� ��� � �<& ���U�U �S�7E�� � ���U�UK� (� ��� �S� E����9���U�U � �7E � ��>

{|^�Z[f�T�Z[f�ioXg~ j W�l[T � ikl�WLj_ac\ U�ac^�TV^�Z�acb (� B � (� : acb$W�j_ac\sU }kT��e~�io\]T_^�l[ioac^�lp\hW�}k}oTVX�Zgf W�^O yFW�^�~ (� � iol W j_ac\ U.ac^�T_^�Z acb (� � � �\�${|^BZ[f�ikl j W�l[T � ikl)WsjVac\ U�ac^�T_^`Z)acb

(� �$� K K K (� �

�� (� :

h i 4 h

Page 27: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

�)f�TVXgT�E � , � K K K , � iol�WsX[T_~���j_T_~e~�T_j_ac\sU�acl[i Zgikac^,�AL > � �ktvw � ikikiP��Zgf�ikl)ik\ U }oikT_l Zgf WYZ � }kioT_l io^MW�^O#) ~�io\]T_^�l[ioac^ W�}2jVac\ U�ac^�T_^`Z (� � acb (� � (� : b a�X�l[ac\]T�3 W�E � � f�T_^ (� �sf W�l�Zga = T W]j_ac\sU�ac^�T_^`Zacb (� � � (� � W�l-� T_}k} W�^�~�Zgf�T io^�~���jVZgikac^ f > U�a�Zgf�T_lpikl io\ U }kioT_l Zgf�T,XgT_l[��} Z:�� f�T_a�XgT_\ x � x�ikl+U�Xgavn`T_~,�

�h�I�CFJ�� ��� f�l,W]j_ac^�l[T8*z��T_^�j_T �B�;=� : ! � � (� B �Hikl,W X[i $ f`Z,ik~�T W�}2acb �#� � � f�ikl�j_W�^ = T l[f�a � ^

~�ioX[T_jVZg} > ��l[ik^ $ Zgf�T U�aclpioZgi nioZ > ik^ x �kt";]W�^�~ x �ktct =?> W�^�WYX $ ��\]TV^�Z,l[io\]ik}kWYX�ZgahZgf WYZ,io^ �� tcy � � x2�E��'TVZ)Z[f�T�T_lpl[T_^�j_T acb2Zgf�T�}kWYZ[ZgTVX)f W�l�W�} XgT W�~ >h= T_T_^Mik^�jVa�X[U�a�X�WYZ[T_~ io^BZgf�T W = avn`T�U�Xga?acb �

�h�I�CFJ�� O#� U&ac\ = io^�ik^ $ Zgf�T�} W�lpZ�UFWYXgW $ X�WYU f�acb$r�� ��y � � ;hW�^�~ � �otvw $ ion`T_l�Whl[T_j_ac^�~ U�Xgazacb�acbT�*?��ik~�ik\ T_^�l[ikac^ W�}oioZ > io^ r�� � �4i ��� �'TVZ ik^ TVl[l[T_^�j_T = a�Zgf�Zg�Xg^�ac^�T�*?��ik~�io\]T_^�l[ioac^ W�}kioZ > acb?f > U�TVX[l[�XgbDW�j_Tik^`ZgTVX[l[T_jVZgioac^,�

�#��� � � f l[io\]ik}kWYX WYX $ ��\]T_^`Z = W�l[T_~Mac^ � �>;��4i � $ i n`T_l Z[f�T

� RP�?�.Jv�I � 5�a�X W�}k} � �.�

E ��� � � (� : � � � �� E � ���g� �6��>

*,� � RP���"��J � � ��� � � �zAONPG � � *,����� fR�.dMT_}k^�i3!`a�nT� d�t8)�der�Pf W�l�\hW�~�T W]~�TVZgW�ik}kTV~MlpZg��~ > acbHZgf�T io^�j_}k��lpikac^Macb a�X = i Z�W�}2nYWYXgikT Z >j_}oacl[�XgT_l)ik^BZ > U�TG�R� ��T�T�� W�\ ik^�T,l[ac\]T�b T WYZg�XgTVl acbPZgf�iol = T_}ka �8ik^�Zgf�T }oi $ f`Z acb Zgf�T_a�XgTV\0x � x �

*,�PO#� � W!`T�� � W ^ W�^�~�}kTVZ�� ��� �\� XgTVlpU/��� ���,� ��~�T_^�a�ZgT]Z[f�T�%'T Xg\hWV� X[T_lpU/��l[ik\ U }oT f�i $ f�T_lpZ� T_i $ f�Z���\]a?~���}kT\� i Zgf f�i $ f�T_lpZ6� T_i $ f`Z��� W ^ �gJ2TVZ� = T Zgf�Tsf W�}ob&l[��\ acb'Zgf�T U�acl[i Zgion`T Xga?a�ZglW�^�~]l[T Z���>� � �R��� ��� � � � � �;� �8� �'J2T Z$( �,~�T_^�a�ZgT Zgf�T b ��}o} l[� = j WYZ[T $ a�X > acb.\]a?~���}kT_l�ik^-(W�^�^�ikf�io} WYZgTV~ =�> W�U�a � TVX�acb�Zgf�T W�� $ \]T_^`Z�WYZgioac^ ik~�T W�} " ) acb Zgf�T j_TV^�Z[X[T#"6��Q�� acb,[\� Q����'��TVZ�� : �� � ��> � � ;���� ����� : � � ����> � � ;�� � �$ � �� � � f�T j_a�X[XgTVlpU�ac^�~�io^ $ j_}kW�l[l[T_l ��� : ����N� : � � ��= a�Z[f�b a�Xg\�� = W�l[T_l�acb�Zgf�T @�X[a�Zgf�T_^�~�ikTVj�! $ Xgac�U acb*( � � {|~�T_^`Zgiob > ��� : �'�)ioZgf8� �� &��<� �� f�T_^ �N� : �2ik~�T_^`Zgi�7 TVl � i ZgfeW�^ T_}oT_\]T_^`Z ���6����� � � � f�T XgTVl[��}oZgio^ $ = W�l[ikl - #���6� �,� � / acb

h i , h

Page 28: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

��� ikl&Zgf�T�ac^�T�\ T_^�Z[ikac^�T_~Lik^et��>; W�^�~B~�T_lpjVXgi = T_~�ik^V�1 �t��E� {�Z f W�l ^�a � = TVT_^L~�TVZgTVXg\ ik^�T_~ =?> Zgf�TZ[Xg�Z[f��1L L�y>L��.��acb2Zgf�T�� W��Vf�~ W�^ ) J ��l[�VZgi $ j_ac^|T_jVZg�X[T_lg���bJ�t��E�

@�ion`T_^ W�^ > 7 ^�ioZ[T_} > $ T_^�TVX�WYZgTV~ [\� Q���\]az~���}oT � ac^�TM\]W > ~�T87 ^�TeioZgl]W�l[l[azjVi WYZgT_~ nYWYXgikT Z > ?���M��& Q ^ � f�l iol6� TV}k}3)]!^�a � ^ �����(� ��& ?�7� ��b a�X W�^ > 7 ^�ioZgTs~�ik\]TV^�l[ikac^ W�} [\��Q���\]az~���}oT�S�,� T_j_W�}k}PZgf WYZ6 `W�^`Zg�_T_^eZ[X�W�^�l[}kWYZgikac^��4a�X,jVacf�TVXgT_^`Z jVac^�Zgio^?� WYZ[ikac^#�)ac^ ( � ikl |��lpZ,ZgTV^�l[a�Xgik^ $ =�>l[ac\]T 7 ^�ioZ[T)~�io\]T_^�l[ioac^ W�}/[R��Q �P\]az~���}oT��8b ac}k}oa"� TV~ =?> U�XgaN|T_j Zgikac^]ac^`Zga)( � ~�T87 ^�T_~ =�> U�X[ik\hWYX >~�T_j_ac\sU�acl[i Zgikac^V�)ioZgf#X[T_lpU�TVjVZ�Zga "6��Q���� � fXgac� $ f�Zgf�T�W = avn`T]io~�T_^�Z[i�7 j WYZgioac^�l ioZ�io\ U }kT_\ T_^�Z[l WXgi $ f�Z W�jVZgioac^ acb � ac^���� �

��TVZ � ���6��� ?��� : ���2{|ZPikl W���^�ioac^ acb�a�X = ioZgW�}�nYWYXgikT ZgikT_lPj_}kacl[�X[T_l �4l[T_Tb�1 `ry � ���N�?b a�X T���W�\ U }kT"�= �Z,ikl�^�a�Z,^�TVj_T_l[lgWYX[ik} > ioX[XgTV~���j_i = }oTN� � fXgac� $ fMZgf�T XgTV\hWYX ! W = avn`T ioZ�b ac}o}ka � l�T���W�jVZg} > W�l�ik^ �� t��Zgf WYZ

�'&�� E �K��#���6� � � � � �e�7E��<&$ �e���6��>��T WYXgT���^ W = }oT�Zgasl[f�a �9Zgf WYZ���� f�ac}k~�l��*,���,� 4�a � W�lpl[��\]TMQ�l[ik\ U }oT acbFZ > U�TM�R� � f�T_^ dMT_}o^�i�!`avn f W�l�l[f�a � ^`� dX;2�Zgf WYZ� � ���6�< �,� �ikl,i X[XgT_~���j_i = }kTN��� {|^�j_ik~�TV^�Z�W�}o} > ?�7� ��� � �+\hW > bDW�ik}2Zga = T ioX[XgTV~���j_i = }oT T n`T_^ io^eZ > U�T\� iob � ikl,^�a�Zik^`ZgT $ XgW�}]� � ��W�\ U }kT_l \hW >L= T XgT_W�~MaN5#b Xgac\ �1 `d�y � � � �E�3� U'ac^�l[T�*?��T_^`Zg} >B=?> lgW > �1 `ry =��kt�x2�2ac^�Tf W�l� �e���6��� ?���b��� � f���l�dMT_}k^�i3!`a�n�a = Z�W�io^�l)Zgf�T> Jv������� G ��GD��N � � ���3( - � Q � �%-:9 7�8� ; 2 � 9 � � # � ���.

E �K� ���6� � � � � ?� E�� & ?���b��>

*,��� � {�ZsiklsW�}oXgT_W�~ > jV}kT WYXsb Xgac\ lgW > �� �xzy � � u� Z[f WYZ ���6� �� �\� �6� io^ $ T_^�T X�W�}]��q�a"�&TVn`TVXsik^Z > U.TF� y ��W��_f�~ W�^ W�^�~ J ��l[�VZgi $ jVac^|T_jVZg�XgTV~B���bJ�;zyH��T_j Z:� � �HZ[f WYZ�T8*z� W�}ki Z > f�ac}k~�l8� � f�iol,T WYXg} >a�U�Zgio\]ikl[\j� W�l�jVXg��l[f�TV~ =?> � W�lpf�i��+WYX�WhW�^�~ �?W�ioZga ��� �Y�E����a�� T lpZ[ik}k}�~�ah^�a�Z-!^�a � ikb3��� f�ac}k~�lik^ ��� r��

*,���#� f�l[l[��\]T&Q]lpik\ U }oT�acb2Z > U.Tb�R� f�}oZ[f�ac� $ fBi Z)iol+^�a�Z > T Z !z^�a"�)^Biob2Zgf�T�Xgi $ f�Z)j_ac^�T_l ~�T87 ^�T_~=�> Zgf�T##���6�� ������ j_acio^�j_ik~�TG�)ioZ[f Zgf�acl[T ~�T�7 ^�T_~ =?> Zgf�TG�\���b��yFZ[f�T j_a�X[XgTVlpU�ac^�~�io^ $ j_T_}k}ol�~�a

h i�F h

Page 29: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

j_acio^�j_ik~�T W�^�~Mio^�~�T_T_~ WYXgT $ ion`T_^ T �U }kioj_ioZ[} > ZgfXgac� $ f Zgf�T � a = ik^�lpac^ )p��j�f�T_^�lpZgTV~ j_a�X[X[T_lpU�ac^�~�TV^�j_TN�dMT_}k^�i3!`a�nT� d�tcy d r� f W�l WYZ[ZgT_\ U�Z[T_~BZgas~�T_l[jVX[i = T�Xgi $ f`Z�j_ac^�TVl�ik^ Z[TVXg\]l acbPZgf�iol j_a�X[X[T_lpU�ac^�~�TV^�j_T�y\]a�XgT+lpU.T_j_i37 j W�}k} > io^sZgT Xg\]l�acb.\]avnio^ $ ^���\ = TVXgTV~ = a:��T_l$ik^slpZ�W�^�~ WYXg~ ZgW = }kT W�� � acb�lpik�_TOV���)f�ikj�f^ WYZg�X�W�}o} > T_^���\]TVXgWYZgT�Z[f�T ?� �6�� � �.� ��W�jVj_a�Xg~�ik^ $ Zga W � a = io^�l[ac^ )p��j�f�T_^�lpZ[T_~ Z > U�T�W�} $ a�Xgi Zgf�\ �

*,�+*,� ���U�U�acl[TF* � % � � �/� lgWYZgiklpb > ��,/. T �I� � f�T_^�b X[ac\ x � xBW�^�~1x �ot";�ioZ�b ac}k}oa"�)l,Zgf WYZ ?����,/. �<& ?���b� � �]f�jVZ[� W�}k} > Zgf�ikl�f W�l�W�^�T_}kTV\]T_^`Z�WYX > U�Xga?acb�^�a�Z[T_~ =�> 4R�?�`UFW�} ZgT_^�lpZgTVik^sW = ac�Z ;cx> T WYXgl�W $ a � � � f�iol�bDW�jVZ�j_ac\ = ik^�T_~\� i Zgf�Zgf�T+~�T_l[jVXgi U�Zgikac^sacbFZ[f�T+X[i $ f`Z$jVT_}k}ol�io\ U }kTV\]T_^`Zgl W�^ a�Xg~�TVXXgT_}kWYZgikac^ ac^�lpZgW�^�~ WYXg~ Z�W = }kT W�� � j W�}k}oT_~ Zgf�T ik^�~���jVT_~sS,��Z a�a�Xg~�TVX-�4io^ nioT8��acbFW�^ W�^ W�}ka $ ac��lPa�Xg~�TVXXgT_}kWYZgikac^�ac^LU�Xgik\]i Zgion`T�io~�T W�}kl+U�acio^�Z[T_~ ac�Z =�> S,��Z a � S&�����zS�TVlpU ioZgT�T WYXg}oikTVX a�U�Zgio\]iklp\ ydMT_}o^�i�!`avn� d�tcy d r�`lpf�a"� TV~ Zgf WYZ�Zgf�ikl a�Xg~�TVX XgT_}kWYZgikac^�ikl�lpZ[XgiojVZg} > � T W!`TVX2Zgf W�^�Zgf WYZ io\ U }kioT_~ =�> Zgf�T�ik^�j_}o��l[ikac^acbPa�X = ioZ�W�}�nYWYXgikTVZ > j_}oacl[�XgT_l8�

*,���#� f�b ZgTVX+�`UFW�}oZ[T_^�lpZgT_io^�y?a�X = ioZ�W�}znYWYXgioTVZgikTVl � ik^�Z > U�T-�G� \hW > = T+nioT8� T_~ W�l�j�f W�io^�l$acb�^�ik}oU.a�ZgT_^`Za�X = i Zgl�� � f�TV^�Zgf�T&!z^�a"�)^La�Xg~�TVX+XgT_}kWYZgikac^�l&ac^�^�ik} U�a�ZgTV^�Z+a�X = i Z j_}oacl[�XgT_l+ik\sU } > W�^La�Xg~�T X X[T_} WYZgioac^ac^ a�X = ioZ�W�}cnYWYXgikTVZ[ikT_l�� � f�iol2iolP~�T_lpjVXgi = T_~ U �XgTV} > j_ac\ = ik^ WYZga�XgikW�}k} > W�lHW�7pj�f W�ik^ a�X[~�TVX 9 ac^�lpZ�W�^�~ WYXg~Z�W = }oT W�� �,� dMT_}k^�i3!`a�n � d�tcy d r� f W�lsl[f�a"�)^ i ZsZga = TBlpZ[XgiojVZg} > l(Z[Xgac^ $ TVXsZgf W�^ Zgf WYZsik\sU }kikTV~ =�>a�X = i Z�W�}�n�WYX[ikTVZ > j_}kaclp�XgT_l��

� f�TVXgT iol�Wg�+W > Zga XgT87 ^�T =�> Zgf�T ik^�~���jVT_~eS���Z aha�X[~�TVX,W�^�~ Zgf�T j�f W�io^Ma�Xg~�TVX�ZgfX[ac� $ f Zgf�T%$a $ W�^ j W�}oj_��}k��l&� j_b �?�1 `ryu��otvw����� dMT_}o^�i�!`avn f W�l$l[f�a � ^]Zgf WYZ$Zgf�T_l[T)XgT87 ^�T_~La�Xg~�TVX[l&lgW�^�~��)ikj�fhZgf�Ta�Xg~�TVX ~�T87 ^�T_~ =�> a�X = i Z�W�} n�WYX[ikTVZ > j_}kacl[�X[T_l��hq�a�U�TVb ��}k} > Zgf�TVl[T]j_ac\ = ik^ WYZ[a�Xgi W�}o} > ~�T87 ^�TV~�a�X[~�TVXglj W�^ = T�lpf�a"�)^ Zgasj_acik^�j_io~�T��

*,���,� 5 io^ W�}k} > }kTVZ � � = T Zgf�Te��^�i�*z��T }kac^ $ T_lpZ�T_}oT_\]T_^`Z�acb#� � f�b ZgT X %$a $ W�^ � %&��ac^�Tef W�lE �K� �,#� �6� � �� ��O O � B � ��O O � : ��5 Xgac\ Zgf�ikl W�l > \ \]TVZ[X > U�Xga�U�T X[Z > acb�- ���6�< �,� � /ik\sU }kikTVl�Z[f WYZ Z[f�T \hWYUK��O O�� : �� ��O O�� : :�� iolsW�^ a�Xg~�TVXsXgTVn`TVXglpik^ $ io^�j_}k��lpikac^�ac^�Zgf�T lpTVZ-"_ �X_6ac]d [R� ��� _ � " ) / �'{|^ $ T_^�T X�W�} Z[f�ikl \]WYU bDW�ik}ol)Zga = T W�^Ma�Xg~�TVX X[TVn`TVXgl[io^ $ ik^`n`ac}k�Z[ikac^TVn`T_^ b a�X�^�ik} U�a�ZgTV^�Z)a�X = ioZ)j_}oacl[�XgT_l_y Zgf�ac� $ f Z[f�ikl ~�azT_l)f�ac}k~ io^BZ > U.T6�R� f $ W�ik^��4ik^�Z > U�Tb�G�&i Z iol^�a�Z !^�a � ^�ikb ?���b�3�� ?���6� ���$ikl W�^Ba�X[~�TVX)X[TVn`TVXgl[io^ $ io^�n`ac}o�Zgikac^�ac^LZgf�T,l[TVZ acbPa�X = i Z�W�}.nYWYXgikT Z >

h i � h

Page 30: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

j_}oacl[�XgT_l��

� = nzikac��l[} > W�}o}Zgf�T_l[T j_ac^�l[ik~�T X�WYZgikac^�l�Zga $ TVZgf�T X �)ioZgfsac�X&\hW�io^ Z[f�T_a�XgT_\ lpZ[Xgac^ $ } > \]a�ZgionYWYZgT_lZgf�T�j_ac\ U �Z�WYZgioac^Bacb Zgf�TG�\� �6�< �,�.� �

���H���zNHEHG���� ��� � �.N F�.AP�2��G ��N �HJv��E��HQ�� � �.JL�HJv�.���JhGDN��v�J��_�Q���G ��N

� � � � � � f�TM^�a�Z�WYZgioac^ f�TVXgTMb ac}o}ka � l � U @ ��yA; � � � J TVZ � 1 � c � t � ;���r � = T l[\ aza�Zgf jVac\ U }kT��W�} $ T = X�W�ikjLn�WYX[ikTVZgioT_l_y�" 1 ��� ��1� � � � �]c � ����� �pt � ; ��� � ;���re� � = T @.WYXgikl !i+j_}kaclpT_~ ioXpXgT_~���j_i = }kTl[� = l[T Zgl�W�^�~ }oTVZ �21 �� � � � � � � � � � 1 � � = T�Zgf�T l(Z�W�^�~ WYXg~ U�XgaN|T_j Zgikac^�l��S,T_^�a�ZgT

" � � ��" � ����� � � � � &��� � �'" � � � +�� &��� � �'" � �N� ��>��T W�l[l[��\]T,Zgf WYZ Zgf�T�X[T_lpZ[XgiojVZgioac^ acb � � � ZgasZgf�T�ik^`ZgTVX[l[T_jVZgioac^

� & �� � �'" � � � + � &��� � �'" � � �<��" � � ��� " � �ikl+U�Xga�U.TVX�l[a Zgf WYZ)ik^BUFWYXpZgikj_��}kWYX�" � � � " � � ikl j_}kaclpT_~ ik^ � � � �Y�

J TVZ�01�� ~�ik\ ��1[y�01 ��� ~�ik\ " 1 � ����T WYX[T�io^�ZgT XgT_lpZgTV~ ik^ Z[f�T�j_ac^`n`ac}k�Z[ikac^ \]WYU

�pt� � �� $ � �'" � � � � � �� ��� �'" � � � � � � �� $ � � " � � ��" � � ��)f�TVXgT

� ; � � � �� � � �� � � � � >��ik^�jVT " � � W�^�~�" � ��WYXgT�ioX[XgTV~���j_i = }oT�y

� �� $ � �'" � � � �!� � " � � ��� � �� ��� �'" � � �<� � � " � � ��>� f?��l_y Zgf�T�j_ac^�n`ac}o�Zgikac^�U�Xgaz~���j ZR�pt�$ikl ~�T87 ^�T_~ =�> Zgf�T�jVazT8� j_ikT_^`Zgl � 1 io^BZgf�T�b a�Xg\���} W

��re� � " � � � K � " � � � ���1 � �

� 1�� ��1��

h ? h

Page 31: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

�)f�TVXgT � � � > > > � � � WYXgT,Zgf�T�ioX[X[T_~���j_i = }oT j_ac\ U.ac^�T_^`Zgl)acb�" � � ��" � � acbH~�io\]T_^�l[ioac^ � �Y�� � � O �

�'�zI�I�C � � ���.( - � 2 ��� � 2 �.��� ��4 2 � �� �<; � & �� � �'" � � � 1 ��2 � � & �� � �'" � �N� � � U�X[a�U�TVX 2 � � 2 � � 2 � � 2 � �4�� - 9 �� ��. 2 �� ; 2 �/� � 2 �.�3� ��4 2 � �� � &��� � �'" � � � + � &��� � �'" � � �

� � ��� *,��- �) �� � �� � � ��� � � � � # � ���. � � 2 ���84�� ��� 4 � �. 2 � � 1 � ���)� � � � 9 ��� ��2 � � � � 2 ��� �)��� #> Jv��� � � f�jVj_a�Xg~�ik^ $ ��5���yMU'a�X:�)tvu��>;���We��y�Zgf�T U�Xga?~���jVZhacbg� � & �� � �'" � � � � � i Zgf � � &��� � �'" � ���E� j W�^ = Tj W�}oj_��} WYZgTV~ ik^ Zgf�T U'f�a � $ Xgac�U ~�T�7 ^�T_~ io^ ��5��4yA=��ot��9��io^�j_T Zgf�T ik^`ZgTVXgl[TVjVZgikac^ iol U�Xga�U�T X y Zgf�TXgT_lp��}oZ�iol W }kio^�T WYX jVac\ = io^ WYZgikac^

� � ���1 � �

1 � � 1 �

�)f�TVXgT �)1 WYXgT�Zgf�T�jVac\ U�ac^�T_^`Zgl&acb � &��� � �'" � � ��+ � &��� � �'" � � ��acb�~�ik\ T_^�l[ikac^ � � W�^�~ 1PWYXgT�U�aclpioZgi n`Tik^`ZgT $ T XglR�|��TVXpXgT � l�io^�Z[TVXgl[T_j Zgikac^M^���\ = TVXgl ��y�l[T_T ��5���y =�� ; � {|^ UFWYX[Zgioj_��} WYX_y 1<� t iob � &��� � �'" � � �+W�^�~� &��� � � " � �Y�'ik^`ZgTVXgl[TVjVZ Z[XgW�^�lpn`TVXglgW�}o} > WYZ � 1 �

4�a � Zgf�T j_ac^�n`ac}o�Zgikac^�U�Xga?~���jVZ&io^<�pt�2iol�Zgf�T j_ac\ U.acl[ioZ[ikac^�acb�Zgf�T j_�U]U�X[az~���jVZ � i Zgf ��� � �N� ^ ���ik^�jVT

� x � � � � � � ^ � � �)1�� � ����� w � ~�ik\ � � � � � 1 � W � �~�T $ � � 1 � � � � � � 1 � �8� � � � � � 1 � �7� ~�io\ � � � � � 1 ���� � � �

Zgf�T W�l[l[TVX[Z[ikac^ acb Zgf�T }oT_\]\hW b ac}o}ka � l��

���H���zNHEHG���� O � � CF�_� QYRPCFN �F� � �.JhQc��N ���AP�2��G �.N �PJ���E��PQc� �� O � � � � RP�?C � �[��RP�?�.Jv�?��GDQ � �.J�I��PADC���G ��N � � � � R2�.I���AD� � � � {(^ Zgf�T lpZg��~ > acb = W�lpTsj�f W�^ $ Tb a�X L+a�X[T_}3) dea?a�XgT�f�ac\]ac}ka $ > ac^�T�f W�l)Zga ��l[T i Zgl = ionYWYXgi W�^`Z W�^�~ l[f�T W�b )4Zgf�T_a�XgTVZ[ikj ^ WYZg�X[TN�

h ?��Bh

Page 32: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

{|^G�)f WYZPb ac}o}ka � l/� T$~�T_^�a�ZgT =�> �F).� HH�FZgf�T$~�TVXgi n`T_~ j WYZgT $ a�X > acb = ac��^�~�TV~ = T_}oa"�1j_ac\sU }kT��T_lacb l[f�T W n`T_l�acb�� n`T_jVZga�X lpUFW�j_T_l�ac^eWs}kazj_W�}k} > j_ac\sUFW�jVZ�lpUFW�jVT�HT� � T_j W�}o}2Zgf WYZ b a�X,W j_ac^`Zgio^?��ac��l\hWYU R H � � � acb�}ka?j W�}k} > jVac\ UFW�jVZ&l(UFW�j_T_l&Zgf�TR� ~�TVXgion`TV~#��b ��^�jVZga�X�R�� �� ) �7HV� � � ) ���g�acb ~�ioX[T_jVZ�io\hW $ T6�)ioZgf U�X[a�U�TVX�lp�U�U�a�X[Z,iol�~�T87 ^�T_~,��dMa�XgTVa�n`TVX_y.ikb R � f W�l�W�7 ^�ioZ[T j_acf�ac\]ac}oa $ ikj W�}~�ik\ T_^�l[ikac^�yFW�Xgi $ f`Z�W�~|acio^�Z)b ��^�jVZga�X

R � �� ) ���g� � � � ) �7HV�ikl�~�T87 ^�TV~,� � f�T = ionYWYXgikW�^�Z f�ac\]ac}oa $ > Zgf�TVa�X > b a�X }kazj_W�}k} > j_ac\ UFW�jVZ�l(UFW�j_T_lsW�l[l[i $ ^�l�Zga We\hWYUR H � � acbHW j_ac\ U }oT����4a�X+Zgf�T j_ac}o}kT_j Zgikac^BacbPioZ[l)f�ac\]ac}oa $ > $ Xgac�U l��

�T��R ��� �Lq�ac\ ����� � R � �����b� � ��)f�TVXgT ��� ~�TV^�a�ZgT_l Zgf�Thj_ac^�l(Z�W�^�Z l[f�T W�b ac^ H j_a�X[XgT_l(U�ac^�~�ik^ $ Zga � y l[T_T�� 5�d ��y�{�� � � r�� L a�XgT_} )dMaza�XgTsf�ac\]ac}ka $ > �T�7HV� ikl W b X�W $ \]T_^`Z acb&Z[f�ikl = i n�WYX[i W�^`Z Zgf�T_a�X > yP^ W�\]T_} > y�ioZ iol ~�T_lpjVXgi = T_~�W�l�T��R �'�)f�TVXgT�R8 H � &�ikl&Zgf�T,\hWYULb X[ac\aH Zga W U�acio^�Z�� � f�T,b ��^�jVZga�XgikW�}ki Z > acb L a�XgTV}3)�dMa?a�XgTf�ac\]ac}oa $ > ikl ~�T_~���j_T_~eb Xgac\ Zgf�iol)io^�Z[TVX[U�XgTVZgWYZgikac^

��� (���� ��; �,�31=�,� * W�^ > U�Xga�U.TVX \hWYU R@ �H � � � $ i n`T_lLXgikl[TeZga W ~�T $ X[T_T �_T Xga�\hWYUR ^ P�T�7HV� � � �T���F� >

��� ( � � 6 � 4 � W�^ > \]WYU4R8 PH � � � 2 �� � 2 ���.� 1 ��2 � � * ���,����� 4) ����� *%�4�T��R �9~�T�7 ^�T_l�W~�T $ XgTVT ]\hWYU R ^. �T���\� � � ��� HH� >

U'} W�l[lpikj W�}o} > j_ac^�l[io~�TVXgT_~ U ��}k} = W�j�!zl]b a�X�L a�XgT_} )�dMaza�X[T f�ac\]ac}oa $ > WYXgT lpU.T_j_i W�} j W�l[TVlhacb�Zgf�Tac^�T ~�T_lpjVXgi = T_~�W = avn`T�y �)ioZgf Z[f�T T_}kTV\]T_^`Z"* = TVik^ $ ~�T87 ^�T_~ =�> W 7pj W�^�ac^�ioj W�}Pa�XgioT_^`Z�WYZgikac^ 9 acb R+�5Fa�X T���W�\ U }kTYy2W�^ > \]a�X[U f�iol[\ R 5H � � acb'lp\]aza�Zgf j_ac\ U }oT��enYWYXgikTVZ[ikT_l�W�~�\]ioZ[l�W�j W�^�ac^�ikj_W�}a�XgioT_^�ZgWYZgikac^�*%�4�T��R �$acb2~�T $ XgT_T�~�ik\ � � ~�ik\ HT�

f�^�a�Zgf�T X�ik\sU�a�X[Z�W�^`Z+ik^ $ X[T_~�ikT_^`Z�acb = ionYWYXgi W�^`Z)Z[f�T_a�X > ikl+Zgf�T b ac}o}ka � io^ $ � J2TVZ

���e�

� � � "

� �?H�

h ? i h

Page 33: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

= T Wsj_WYX[ZgT_l[ikW�^ ~�ikW $ X�W�\ � � f�TV^MWs\hWYU+R ^ P�T� � � � � �T��� �$iol)~�T87 ^�TV~,�� O � O � � f�T)b ac}k}oa"�)ik^ $ XgT_l[��} Z ikl'W�~�ioX[T_jVZ j_ac^�lpT�*z��T_^�jVT�acb.Zgf�T = W�lpT�j�f W�^ $ T Zgf�T_a�XgTV\ b a�X }oazj W�}o} >j_ac\ UFW�j Z Z[a�U�ac}ka $ ioj W�}�lpUFW�j_TVl_y�lpT_T � ����j�f���y 3$X[a�U/� ; � x �ktct��

�'�zI�I�C � �]L W�lpT j�f W�^ $ Tsb a�XIL+d f�ac\ ac}ka $ > � � �5( 9 9 � � � 2 � � 2 � 2 ��� 4 �,� 2 �.� � �, *,� ���,� � - ��� �2 ��� - � 9 R � � 9 ��� 9 �.� #�� � 2 *� �T� � � �, * * �. �� 2 ��� ��R ^ ��* �$# � ���. �|��� � ^. R ^ �(0 ^ � ^� P�T���g� � � �T�'"&��>

� O � �/� �B�H�PAOGDQcC���G �.N �v�1Q`��N ���AP�2��G �.N �HJv��E��HQ�� �

J T_\]\hWIfb; �>; iol W U �X[T_} > b a�Xg\]W�}�j_} W�io\ �P{�Z = T_jVac\]T_l+\]T W�^�io^ $ b ��}�ik^LZgf�T,j W�l[T�Zgf WYZ = a�Z[f�*�W�^�~� j_a�X[X[T_lpU�ac^�~BZga 7pj W�^�ac^�ioj W�}�a�XgikTV^�Z�WYZ[ikac^�l 9 acb ��W�^�~ acb �]XgT_lpU.T_jVZgi n`T_} > �

J TVZ � 18� " 1 � = T W�l ik^Kf t��kt���J2T ZFR 1� � �1 � � � 1 = T \]a�X[U f�iol[\]lR�)ioZgf � �1 l[\]a?a�Zgfj_ac\ U }oT��BW�} $ T = X�W�ikj,n�WYX[ikTVZgioT_l���S,T87 ^�T

" �1 � �k��R 1 4R � � &�� � " 1 � ��>

��Tb� iol[fBZgasj_ac\ UFWYXgT,Z�� a jVac^�n`ac}o�Zgikac^�U�Xgaz~���jVZ[l_y

�E=e� �T�'" � � � � �T�'" � �N� � � �T�'" � � � " � �Y�W�^�~

��ue� ��� " �� � � � �T�'" �� � � � � ��� " �� � ��" �� � ��>� f�T \]WYU l6R 1 R � � �]c � ���>� �pt � ; � a�XF� ;���re����W�~�\ ioZ,Whj W�^�ac^�ioj W�} a�XgikTV^�Z�WYZ[ikac^el[ik^�jVT�Z[f�T_l[TsWYXgT\hWYUBacbHl[\]a?a�Zgf�a�XgikT_^`ZgT_~ \hW�^�ikb ac}o~�l�� � f���l_y Zgf�T io^�~���j_T_~M\hWYU l

R 1��# " �1 � � � " 1��

h ?�? h

Page 34: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

W�~�\]i Z W�^Bik^�~���j_TV~Ma�XgikT_^`Z�WYZgioac^,� � f���l_yFac^�T f W�l)Z[f�T�\hWYU l

�ptvwe� R ^1 � �T�'" 1 �N� � � ��� " �1 � � � �]c � �����k�pt � ; �$a�XI�E;���re��>

� f�T�io^�n`TVX[l[T io\hW $ T�\hWYU l �ptvwe�$j W�^ = T�~�T_l[j_i = T_~MW�}ol[a ��lpik^ $ Z[f�T�io^�n`TVX[l[T io\hW $ T�\hWYU l ik^�j_acf�a)\]ac}oa $ > ycikb�ac^�T�io~�T_^�Z[i�7 T_l�� ^ �'" 1 � �+� i Zgf Zgf�T�jVacf�ac\]ac}ka $ > � � & ^ � � 1 � � � � 1 � � � " 1 � ��W�^�~l[io\]ik}kWYXg} > b a�X �T�'" �1�� �����io\]ik}kWYXg} > y�Zgf�T \hWYU+R � 4R � � �� � �� � � � � � � ik^�~���j_TVl Zgf�T�\hWYU

R ^� � P�T�'" � � ��" � �N� � � �T�'" �� � � " �� � ����)f�TVXgT�R � � ikl+Zgf�T,XgT_lpZpXgikjVZ[ikac^ acbZR � 4R �Y���Tb� io}k}�lgW > Zgf WYZ = W�l[T j�f W�^ $ T f�ac}o~�l b a�X Zgf�T jVac^�n`ac}o�Zgikac^�U�Xgaz~���jVZ ikb Zgf�T ~�ikW $ X�W�\

�ptct�

�T�'" � � � � ��� " � � � �F�T�'" � � ��" � � �

�T�'" �� � � � ��� " �� � �

��$��� �����

�F�T�'" �� � ��" �� � �

��$�

j_ac\]\��ZgTVl��

> Jv������� G ��GD��N � �5( 9 9 ��� � R � � �� � � � � � � �, � � � - �,� 9 � � � - # � ���) 6 ��� � 43� � � ����, * � ; �,�2 ��� 4��� � �,&( 2 � �, 9 ��� * (54 2 #> Jv��� � � � f�T jVac^�n`ac}o�Zgikac^ U�Xgaz~���jVZ'ikl$~�T87 ^�T_~LW�l&W�j_ac\ U.acl[ioZgioac^sacb�Zgf�T io^�n`TVX[l[T io\hW $ T �#^ � � � �#^� �W�^�~MZgf�T ~�ioX[T_jVZ,ik\]W $ T � � � ^ � � f�T io^�n`T Xgl[T ik\]W $ T a = nikac��lp} > jVac\]\��ZgT_l �)ioZgf Z[f�T = W�l[T j�f W�^ $ TN�� f?��l)ioZ XgT_\hW�io^�l Zga j�f�T_j ! Zgf WYZ Zgf�T�}kT_b Z l *z� WYX[T�acbPZ[f�T�j_ac\ \ �ZgWYZgion`T�~�ikW $ X�W�\

�pt"; �

" �� � � �� " �� � � � $

�� " �� � � " �� � � � �� � ��

" � � �� � " � �

$��� ���

� $�� " � � � " � �

$�

� � � � �

$� �

h ? � h

Page 35: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

ik^�~���jVT_l Zgf�T�j_ac\]\��Z�WYZgi ni Z > XgT_}kWYZgikac^

��� � � � � ^ ��R ^� � R ^� � ����R ^� � � � � � � ^ >

��ik^�jVT�R � ikl W�^ iol[ac\]a�X[U f�iol[\ yZgf�T�}kT_b Z j_ac\]\��Z�WYZgi n`T�l *z� WYXgT�ik^ Z[f�T ~�i W $ XgW�\ �(t"; �'iol j WYXpZgT_l[ikW�^,�� f�TVXgT_b a�XgTYy =�> J T_\]\hWRfb; � ;zye� T,f Wvn`T |��lpZ Zga j f�TVj�!LZgf WYZ Z[f�T�j W�^�ac^�ioj W�}�a�XgioT_^�ZgWYZgikac^�l ��l[TV~ b a�Xj_ac^�lpZpXg��jVZgioac^ acb2Zgf�T�ik^`n`TVXgl[T�ik\hW $ TVl R,^� � R,^� � W�^�~4R,^� � y�WYX[T�j_ac\sUFWYZgi = }oTN�� f�ikl W�\]ac��^�Z[l Zga j�f�T_j !io^ $ Zgf WYZ ik^ Zgf�T�j WYX[Z[T_l[i W�^ ~�i W $ X�W�\

� �� �� �� � �� � � $�� � �� � ��

� � �� � � �

$� � � �

� $�� � � � �

$� �

Zgf�T j W�^�ac^�ioj W�}Ha�XgioT_^`Z�WYZgikac^�l acb_R � R � R �sW�^�~ acb<R � R � WYXgT j_ac\ UFWYZ[i = }kT� � f�ikl�ikl�a = nioac��ll[io^�j_T�R � iol�W�^Bikl[ac\]a�XpU f�ikl[\ �

�h��� �zJ��zNHQc��

��L-L � fR��L T_io}kio^�l[ac^1W�^�~ �+L+T Xg^�lpZgT_io^�y � � 4 � � �$� 2 � �� * � Q - � * (�7�3� y U'� f �Fy�3�WYX[ikl_y'���TVX��Xf yO�� O �ptvu�=t��y.t�x��t =��

��LM� � � L+TV^�}kac}ka�y �5(�� � ')(/�, 2 � �"4 � 2 � �� * � 43�.� 2 � � ��.� � �,� ���� 2���3� ��� 6.��2 � 7�3� y/L+��}o}]� ��j_i]� d WYZ[f,�� �ptvucu � ��y�;�;cx��; ��t��

�1L @?� { �C4R� L+T Xg^�lpZgT_io^ W�^�~ ��� {���@�T_}obDW�^�~�y �0�. �� ��� 9 ��� * (54 2 � � ; � ��2 ��� * � � ��2 � *,�%- �. �� � �� 0� � � 9 � �3� �. 2 � 2 � �� �� �<; � � - � � �%-:9 7� � � � � ��� 6 � �,� y#U'ac\ U�acl8�$deWYZgf,� � � �(tvu�=cwe��y ; ��x��;N=`x ��1L L+d � �k�'L a�Xgf�a�y �D) J ��L&X > }kio^�l !i'W�^�~��6� deW�j 3'f�T Xgl[ac^�y�� � 9 � 2 �) 2 ��� 6)��2 � � 9 � ��- �%2 � ��� � * � � ��, * 43� �,� � 4 2 �.� � � 2 � 44$ ����� �3� y�3�Xga $ X:�$deWYZgf,�+�2=y L i X !f��W���l[T X y�L+acl(Zgac^�y�tvu�=cu��

h ? ` h

Page 36: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

�1L��b� �D)EJ � L X > }kio^�l !iFW�^�~�dH� ��W�l[f�i�� WYX�Wy�� ���.� * �, �� � (���� 2 � � 43�� �� ��4 2 (�� � �, * ���,7�� � - � 4:� ��� �2 � - � y�{(^`n`T_^`Z:�&deWYZgf,� * � �(tvu�=t��y�r�=?� � ��tvw���1LM4?� �-L+TVj�! W�^�~ qR� 4,W!YWN|io\hWy�� � ��� 2 � 6 ��� �3� �, * 2 1 ���� � * � * 4��$� � �<; ')(/�, 2 ( - � � ��� � � 6 � ��� y WYX,ion �\hWYZ[f,��� f� 6wY;zt";�;cx�ry ;�wcwY; �� UA� �G� � � U&WYX[ZgTVX_y � ��-:9 7� �,� �,( 9 � �<; � � � 2 � 9 � y�{|^`ZgTVXgl[jVikT_^�j_TYy cacf�^ � ik}kT > y J2ac^�~�ac^#tvu?�; �

�CU-@?� 4R�2U'fXgiklplPW�^�~b%\�2@�io^�� = �X $ y�� � 9 � �3� �) 2 � 2 � �, 2 ���3��� � �, * 4�� -:9 7� � � ��� - � 2 � � y2L ioX !zf��W���l[TVX yL aclpZgac^�tvucu?� �

� S,� = � fR�FS,� = l[ac^�y � ��� - ( 7� 9 ��(��7�3�84 � 4)7�3� �� �,�, ��.� 4��. 2 � y#U&� f �Fy 3�WYXgiol O�� � y ���T X {6�ptvu�= � ��yt =Y;��t = � �

� S.� dH�)S,��Z a�y �5(�� � 4) ���3� � �"4 � 2 � �, * �3� � * �� �,( � 9 � �%- ��2 � ;�� * �, /� �� � ���� 6 � � �) �� �)7� 9 9 �, 2 �* � (� ���� ���� 6 ��� * � � � � � � - � ��� �%-:9 7� y f�^�^,�&deWYZgf,� � � � �ptvu?�c����y�tvw?� ��t";�w����5�� �k� 5F��}oZ[ac^�y � 2 �.�3� ��4 2 � �� � ������� � y��`U�Xgik^ $ TVX_y L TVX[}kik^�tvu�= � �

� 5�d � �k� 5F��}oZ[ac^�W�^�~#�G�2deW�j 3'f�T Xgl[ac^�y�� � 2 ��� �,� � 4 ���;3� � - �.1 ��� � ; ��� 2 �/� � 2 ( * � �<; � � �,(� � �� 9 ��4��3� ��dMT_\ acioXgl acb+f,d �Fy#; �`r �ptvu�=t� ��C@?� %\�&@�ik^�� = �X $ y6Q � - � * (�7�3� � � 9 � � � �.� � � 9 � �3� �) 2 � 2 � �, �� �, * 6.� �,�,� � �, 2 � ���)�3 4$ ����� �3� y

f�~n ��deWYZgf,� */� �ptvu�=c�e�gy�t � �Y=��� q?� �G�zq�a�Z[Z�Wy �" ��� � � 9 ��� � 4��� �� 2 �3(54 2 � �� �<;�� � � �,� �,( 9 ��� 9 ���3� �. 2 � 2 � �� �� y � �acf�aN!z��d WYZ[f,�

�ky �2* �ptvu�= � ��y �`u�� � � ��� t�� fR�e `aclpTVU f�y � � - � * ( 7�:� 2 �.(/4 2 (�� � � 2 ��� 9 � ��- ��2 � � �:� 9 �34 2 �.( - �<; 2 �����) �� �)7� 90� � � � � 6 � ��<; � � � - � � ��-:9 7� � � ��� � � 6 � � y J 4 �;N=y.�`U�Xgio^ $ TVX y L+T Xg}kio^ tvu?�6uy�tctY� ��tvr`x ��� e;2� fR� cacl[TVU f�y�� � -:9 7� 2 � �, ;3(� �4 2 ����� � 2 ��� ( 4 � 2 ������� � y J 4 tvwY;�wy�`U�Xgik^ $ TVX_y�L TVXg}oik^ tvu�=cry

=cw���tvwc���

�� �r� fR� `aclpTVU f�y �" 2 ��� � �,� � � 2 � �<;8� � � �����3� 2 1"� � ��� 2"- � * ( 7� y# � f�} $ T = XgW �&� �ptvu�= � ��y#;�r�=��;��2=��

h ? 4 h

Page 37: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

�� �N� fR�� cacl[TVU f�y �" 2 ���84�� 4) � 4 ��2 � � ; � ��4 2 ���3� ����� � 4 � � 2 � * 1 �%2 ����( � � � �� �,&( 2 � �� �� y J 4 t"; �`ry�`U�Xgio^ $ TVX y L+T Xg}kio^�tvu�=`xzy�t � � ��t =�=��

�� ?x2� fR� cacl[TVU f�y � 2 ��� 43� � � � 4 2 �.� � � 2 � 4 9 �, �, �� - � � � �<; ��� 6.�%2 � �,�,� � � 2 � �3� y�f�^�^,� ��'jN� 4�a�Xg\ ����U/� O�O �ptvu�=cue��y x��cu��z�cwcr��

�� ��� fR� cacl[TVU f�y � � 6.��2 � �,� � � � 2 � �3� � ; 2 ��� - � ��- � ��,� 6.��2 y �,� �(tvucu�=e��y�tY� � ��x ��1 `d � fR�� cacl[TVU feW�^�~`fR�FdMT_}k^�i3!`a�n.y�� (/�, 2 � � � 2 � �� �<; � � 9 �.�3�3(���;$� 43� ��� 6)��2 �� �,�,� � � 2 � �3� y�3$Xga $ X:�

deWYZgf,� L+i X !f��W���lpTVX y�L aclpZgac^`;�wcwcry�tv�`x��tvuc���

���.� dH� � W�l[f�i3�+WYX�Wy � ��� 2 � - � � ; - � 4.��� *,��� �.� �. 2 � �� � ')(/� 2 � �� �� y L ioX !zf��W���l[TVX y�L aclpZgac^�tvu�=cr����� �Y� dH�'��W�l[f�i3�+WYX�WhW�^�~ �F���?W�ioZga�y�� �3� - � 2 � � 4 4��� /� 2 �.(/4 2 � �, �<;84.� ��� 2 � 6 �,� �3� y S���!`T d WYZ[f,�

� � � �ptvucu?����yu��zrc���

��� ��j�f��LdH� � W�l[f�i3�+WYX�W]W�^�~-3'����j f WYU i X�Wy �5��� � � �.� �� - � � ; �, * � y.�`U�Xgio^ $ TVX y L TVXg}kio^ tvucu � ����bJ�t�� S\�CfR��� W��_f�~ W�^ W�^�~ @I��J ��l[�VZgi $ y � � 9 ���3� �. 2 � 2 � �� /� �<; � � ��� 2 �.� �,����( 9 ���, *� ��4 ��� � ��� �6 � ��� y{|^�n`TV^�Z:�'deWYZgf,� ��� �ptvu?�6ue��y�tv�`x��t = � ����bJ+;2� S\�CfR� ��W��_f�~ W�^ W�^�~\@R� J2��lp�VZgi $ y � 2 � 9 �,7� � � 4 � � 9 9 � � � 43� 2 � � 9 � � ���.� � � � � 9 ���3� �. 2 � 2 � �� /� y

f�~n ��deWYZgf,� �&� �ptvu�=cwe�gy�;�;�;��;�;N=��

� deW�j8� �G�$deW�j 3'f�T Xgl[ac^�y � ���.�3 4) ����� �3� ; ��� � � �,(� � � �,� � � � 2 � �3� y f�^�^,� deWYZgf,� ��� � �ptvu?� � ��y� ;�r�� �`rY; �

� d`� fR�HdMT_}o^�i�!`avn�y� ��� - � 2 � � 4 � 2 �.� 9 � � 2 � 2 � �� � ; � � 6.� �� �� �� � 4.���. �� 2 � *89 � � 4�� * (���� �, * ��� � � 2 � * �,� * �.�3��� �����(� � 2 � 6 7� �,( � y � f�T_l[iol_y ��T_ik�V\hW�^�^B{|^�lpZgi Zg�ZgT�y�tvucuY; �� d1t�� fR��dMT_}k^�i3!`a�n.y � � 6)� �� �� � � 43���) �� 2 � * 9 � � 4�� * (���� �, * 4�� - 6.� 0� 2 ��� � � 9 � � 9 �.� 2 � �3� �<; ������- � 2 � � 4 ��� * �.�� �� �7O+��y#U'� f �Fy 3�WYXgiol �,��� y�� �TVX:�${&�ptvucuY; ��yFx�r��x�� �� d ;2� fR��deTV}k^�i�!`avn�y � �3� � * (/4 �+6.� ��2 � �<; 2 �/� ����� � 4 � � 2 � * �,�,� � � 2 � �<; � ��- 9 7� � � �����3� 2 1"� � ��� 2 - � * �( 7�3� � � �� �7O+��y#U'� f �Fy 3�WYXgiol �,� * y�� �TVX:��{.�ptvucucre��yFx�r��x�� �� d r� fR� dMT_}k^�i3!`a�n.y � ��� 6.��2 � �,�,� � � 2 � 4$7� �3(�� �3� � � �� �7O+� � � � � �

h ? , h

Page 38: University of Haifamath.haifa.ac.il/hinich/WEB/mypapers/ovc.pdfN Jv E HQ G .N fTW ik\;acb Zgfiol$UFWYU T X ikl Zga UXgavn`TW Z T_^`Z > T WYX'ac}k~]jVac^ |T_jVZg XgT `r y u u Fac^]io^j_}k

��3 � � ��3 T XgT_}k\]W�^�y y � f�TVl[ikl_y���TVik�_\hW�^�^ ;�wcwcr��

� � T�� dH��� T_io^�T8!`T�y � ( � ���.��� � * �3� � �,(� � � � � � 2 � �, �� �, * 4 � ��� � 4�� 6 ��� �3� y 3�Xga $ X:��deWYZgf,� O#� � yL ioX !zf��W���l[TVX y�L aclpZgac^<;�wcwcryFrY;cx�zr � � �

� �-� �k�H� acl[lp\hW�^�^�y � ��,�,� � �, 2 � � � �. * � � 2 � �+6 ( 2 � �� �� �, � � � -8� � ��-:9 7� � � � � � � 6 � � � * �/��- �,7� � � 4) ����� �.� �� 2 ��� 4��� ��� - �� �,�,� � � 2 � y{ y�{[{ y � 5F��^�jVZ:� f�^ W�}]� �2* �ptvucut��y�tvrcw���t�x ��yt�xcx��tvucr��

� �Ft�� 4R�'�`UFW�}oZ[T_^�lpZgT_io^�y �" 2 ��� � � � * 9 � � 2 � � 2 �<; � (� �&9 � 2 �. 2 �)7� - �) 2 �, 2 ��� � �,� � � 2 � �<;� �����)��3( 6 �,����( 9 � y � a�U.ac}ka $ > � * �ptvu?�c����y ;�wcr���;�w � �

� � ;2� 4R�H�`UFW�}oZgTV^�lpZgT_io^�y � �,��� �3� (� ��9 � 2 �. 2 ��� � 2 � �,( � � �,� ��( 9 �3� * � � �����) y�J 4 u �`�y �`U�Xgik^ $ T X yL TVXg}oik^�y�tvu�=Y; �

� � � � � � W�^�iklgW!zi4y � � �,� ��4 2 �)� � � 2 � 4 �,�,� � � 2 � �3���<;:� � �����.� 2 1 � � ��� 2 - � * (�7�.� �, * 9 � ��- ��2 � � ��')(5��2 � �. 2 � y f�~n � �`Zg��~�ioT_l 3$�XgT deWYZgf,�oy � � �ptvu�=�=e��y�t �zrcw��� %&� dH�+%$TVX $ ^�T�y � �, �� ��� - �3� * � � � � � 9 � � 2 � � 9 � ��3� �. 2 � 2 � �� �� * � � 9 � � � �.� y'f�^�^,����jVi]� ��'j�

4�a�Xg\ �'���U/� O � �ptvucucwe�gyFx �`r2)|x��Y; �

� %'aN� S\� %$a $ W�^�y �"� * �.� � � � 2 ��� 9 � ��- �%2 � ��� � 9 ��4 2 �3( - � ; � � � - � � ��-:9 7� � � � �� � � 6 � � y.d WYZ[f,�f�^�^,� O ��� �ptvu�=cwe��y�tvu`x��;�wcr��

h ?�F h