group theory, representations and their applications in...
TRANSCRIPT
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Group theory, representations and their applications in solid state
TMS 2018
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Why group theory?Powerful: Lets you extract all the physical consequences of the symetries of a system, in a systematic and reliable way
Simple: At the user level, it is one of the simplest, least demanding theoretical techniques
(Besides the four rules, you just need the most basic acquaintance with matrices)
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What symmetries?
Internal symmetries: SU(2) isospin, SU(2)xU(1) electroweak, SU(3) color,...
Space-time symmetries: Rotations, translations, parity, TRS, Lorentz boosts, Galileo boosts,...
In condensed matter we will be concerned with space-time symmetries (parity, TRS, rotations and translations).
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What symmetries?
Discrete symmetries: Discrete groups (characters, orthogonality of characters of IRs, projectors,...)
Continuous symmetries: Lie groups (Casimirs, Cartan subalgebra, Dynkin diagrams, Young tableaux,...)
In condensed matter we will be concerned with discrete symmetries, more concreteley with discrete subgroups of O(3), lattice translations and TRS.
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What consequences?
Discrete symmetries: Symmetries ➞ Degeneracies + selection rules,...
Continuous symmetries: Symmetries ➞ Conservation laws ➞ Degeneracies + selection rules,...
It is relatively easy to obtain physical consequences from conservation laws, even without (explicitly) using group theory.
In the absence of conservation laws, the use of group theory to obtain the consequences of the system symmetry is virtually unavoidable.
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Continuous symmetries
Conservation laws
Degeneracies, selection rules, ...
Discrete symmetries
Degeneracies, selection rules, ...
GT
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Applications of Group Theory in Quantum Mechanics M I Petrashen, E D Trifonov
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Outline of the course
1. Introduction: Symmetries, degeneracies and representations.
2. Irreducible representations as building blocks. Application to molecular vibrations.
3. Operations with representations: Physical properties and spectra.
4. Spin and double valued representations. Splitting of atomic orbitals in crystals.
5. Representation theory and electronic bands.
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Outline of the course
1. Introduction: Symmetries, degeneracies and representations.
2. Irreducible representations as building blocks. Application to molecular vibrations.
3. Operations with representations: Physical properties and spectra.
4. Spin and double valued representations. Splitting of atomic orbitals in crystals.
5. Representation theory and electronic bands.
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Vibrational spectrum of methane (CH4)Can we say anything about the spectrum just by looking at the molecule?
In the harmonic approximation, the frequencies and normal modes can be found by diagonalizing a 15x15 symmetric matrix.
Diagonalizing a generic 15x15 matrix yields 15 frequencies
4+1=5 atoms5x3 =15 degrees of freedom
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Vibrational spectrum of methane (CH4)BUT: Our 15x15 matrix is not generic. It describes the dynamics of a molecule.
The 15 degrees of freedom include 3 rigid translations and 3 rigid rotations, where the molecule moves as a whole without changing the geometry of the bonds. This leaves 9 genuine vibrations.
That is as far as we can go without taking into consideration the symmetries of the molecule.
15=6 zero modes + 9 vibrational modes
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If you know group theory, you can easily obtain:
You will also know that all 4 frequencies are Raman active, but only the triply degenerate frequencies are IR active.
Moreover, methane is MW inactive.
1x1+1x2+2x3=9(1 non-degenerate +1 doubly +2 triply degenerate = 4 different frequencies)
Vibrational spectrum of methane (CH4)
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Moreover, deuterated methane is MW active.
Vibrational spectrum of CH3D
Group theory tells you that each triply degenerate frequency splits: 3 ➞1+2:
Now all 6 frequencies are both Raman active, and IR active.
1x1+1x2+2x3=9➞3x1+3x2=9(3 nondegenerate +3 doubly degenerate = 6 different frequencies)
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Vibrational spectrum of methane (CH4)GROUP THEORY COMPUTATIONS:
These are all statements about representations.
A representation tells you how something transforms under the symmetries of the system.
M = A1 + E + 3T2 + T1
V = T2
R = T1
V ib = M � V �R = A1 + E + 2T2
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Vibrational spectrum of methane (CH4)GROUP THEORY COMPUTATIONS: M = A1 + E + 3T2 + T1
V = T2
R = T1
V ib = M � V �R = A1 + E + 2T2
M: Mechanical representation. dim (M) = 15V: Vector representation. dim (V) = 3R: Rotational representation. dim (R) = 3Vib: Vibrational representation. dim (Vib) = 9
15=6 zero modes + 9 vibrational modes
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Vibrational spectrum of methane (CH4)GROUP THEORY COMPUTATIONS: M = A1 + E + 3T2 + T1
V = T2
R = T1
V ib = M � V �R = A1 + E + 2T2
are irreducible representations of the point group Td(4̄3m){A1, E, T1, T2}
IR A1 E T2 T1
dim 1 2 3 3
V ib = A1 + E + 2T2
9 = 1⇥ 1+ 1⇥ 2+ 2⇥ 3
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If you know group theory, you can easily obtain:
1x1+1x2+2x3=9(1 non-degenerate +1 doubly +2 triply degenerate = 4 different frequencies)
Vibrational spectrum of methane (CH4)
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Vibrational spectrum of CH3DGROUP THEORY COMPUTATIONS:
M = 4A1 +A2 + 5E
V = A1 + E
R = A2 + E
V ib = M � V �R = 3A1 + 3E
IR A1 A2 Edim 1 1 2
are irreducible representations of the point group C3v(3m){A1, A2, E}
V ib = 3A1 + 3E
9 = 3⇥ 1+ 3⇥ 2
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The symmetry groupThe symmetry group of CH3D is .C3v(3m)
is the group of symmetries of a triangular pyramid.
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In group theory language, we say that ~a belongs to thevector representation V and write
V (ax
, ay
, az
)
Invariants can be constructed by taking inner products of objectsthat belong to the same IRREDUCIBLE representation, sothat their variations cancel out.
System invariant under the crystal point group C3v.The hamiltonian H(~a) depends on one vector variable.
~a = (ax
, ay
, az
)
vskip1cm The pyramid is invariant under 2⇡3 rotations
and 3 vertical reflection planes.The hamiltonian H(~a) depends on one vector variable.
The pyramid is invariant under 2⇡3 or 4⇡
3 rotationsand under 3 vertical reflection planes.
V (C+3 ) =
0
@� 1
2 �p32 0p
32 � 1
2 00 0 1
1
A , V (�a
) =
0
@�1 0 00 1 00 0 1
1
A
The group C3v is generated by a 2⇡3 rotation around
the OZ axis and a vertical plane:
The vector representation V decomposes into the twoirreducible representations (IRs) A1 and E
V (ax
, ay
, az
) = A1(az) + E(ax
, ay
)
az
a2z
, a2x
+ a2y
a3z
, az
(a2x
+ a2y
)
C3v
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The vector representationTells you how ordinary vectors transform under the elements of the symmetry group:
C3v = {E,C+3 , C�
3 ,�d1,�d2,�d3}
�d1
�d2
�d3
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V (E) =
0
@1 0 00 1 00 0 1
1
A
V (C+3 ) =
0
B@� 1
2 �p32 0p
32 � 1
2 00 0 1
1
CA
V (C�3 ) =
0
B@� 1
2
p32 0
�p32 � 1
2 00 0 1
1
CA
V (�d1) =
0
@1 0 00 �1 00 0 1
1
A
V (�d2) =
0
B@� 1
2 �p32 0
�p32
12 0
0 0 1
1
CA
V (�d3) =
0
B@� 1
2
p32 0p
32
12 0
0 0 1
1
CA
The vector representation
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V (E) =
0
@1 0 00 1 00 0 1
1
A
V (C+3 ) =
0
B@� 1
2 �p32 0p
32 � 1
2 00 0 1
1
CA
V (C�3 ) =
0
B@� 1
2
p32 0
�p32 � 1
2 00 0 1
1
CA
V (�d1) =
0
@1 0 00 �1 00 0 1
1
A
V (�d2) =
0
B@� 1
2 �p32 0
�p32
12 0
0 0 1
1
CA
V (�d3) =
0
B@� 1
2
p32 0p
32
12 0
0 0 1
1
CA
The vector representation
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�d1
�d2
�d3
X
Y
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Definition of representationA representation T of a group G assigns to each element g ∈ G a linear operator T(g) in such a way that
g1 � g2 = g3 ) T (g1)T (g2) = T (g3)
0
B@� 1
2 �p32 0p
32 � 1
2 00 0 1
1
CA
0
B@� 1
2 �p32 0p
32 � 1
2 00 0 1
1
CA =
0
B@� 1
2
p32 0
�p32 � 1
2 00 0 1
1
CA = V (C�3 )
Example:
C+3 � C+
3 = C�3 ) V (C+
3 )V (C+3 ) = V (C�
3 )
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Reducible and irreducible representations
V (E) =
0
@1 0 00 1 00 0 1
1
A
V (C+3 ) =
0
B@� 1
2 �p32 0p
32 � 1
2 00 0 1
1
CA
V (C�3 ) =
0
B@� 1
2
p32 0
�p32 � 1
2 00 0 1
1
CA
V (�d1) =
0
@1 0 00 �1 00 0 1
1
A
V (�d2) =
0
B@� 1
2 �p32 0
�p32
12 0
0 0 1
1
CA
V (�d3) =
0
B@� 1
2
p32 0p
32
12 0
0 0 1
1
CA
V (x, y, z) = A1(z) + E(x, y)
13
In group theory language, we say that ~a belongs to thevector representation V and write
V (ax
, ay
, az
)
Invariants can be constructed by taking inner products of objectsthat belong to the same representation, so that their variationscancel out.
System invariant under the crystal point group C3v.The hamiltonian H(~a) depends on one vector variable.
~a = (ax
, ay
, az
)
vskip1cm The pyramid is invariant under 2⇡3 rotations
and 3 vertical reflection planes.The hamiltonian H(~a) depends on one vector variable.
The pyramid is invariant under 2⇡3 rotations
and 3 vertical reflection planes.
V (C+3 ) =
0
@� 1
2 �p32 0p
32 � 1
2 00 0 1
1
A , V (�a
) =
0
@�1 0 00 1 00 0 1
1
A
The group C3v is generated by a 2⇡3 rotation around
the OZ axis and a vertical plane:
The vector representation V decomposes into the twoirreducible representations (IRs) A1 and E
V (ax
, ay
.az
) = A1(az) + E(ax
, ay
)
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A representation T of a group G assigns to each element g ∈ G a linear operator T(g) in such a way that
g1 � g2 = g3 ) T (g1)T (g2) = T (g3)
0
B@� 1
2 �p32 0p
32 � 1
2 00 0 1
1
CA
0
B@� 1
2 �p32 0p
32 � 1
2 00 0 1
1
CA =
0
B@� 1
2
p32 0
�p32 � 1
2 00 0 1
1
CA = V (C�3 )
Example:
C+3 � C+
3 = C�3 ) V (C+
3 )V (C+3 ) = V (C�
3 )
Reducible and irreducible representations
C+3 � C+
3 = C�3 ) E(C+
3 )E(C+3 ) = E(C�
3 )<latexit sha1_base64="nymKrHjdrn0N2LZuD3dhXJghRd4=">AAACIHicbZBbSwJBHMVn7WZ22+qxlyEJlFB2u/sgSBL0aJEX0G2ZHWd1cPbCzGwh4pepL1MQRPVUn6ZxVajs/zK/OecMzPk7IaNCGsanlpibX1hcSi6nVlbX1jf0za2aCCKOSRUHLOANBwnCqE+qkkpGGiEnyHMYqTu98siv3xEuaODfyH5ILA91fOpSjKSSbL1Ytg9v91uYcgxjhLGSg61r2ulKxHlwDy8ysZWdnsUx5LK2njbyRjxwBMeFkwI0p8oU0mAyFVt/abUDHHnEl5ghIZqmEUprgLikmJFhqhUJEiLcQx3SVOgjjwhrENccwj034FB2CYzvP7MD5AnR9xyV8ZDsir/eSPzPa0bSPbMG1A8jSXysIspzIwZlAEfbgm3KCZasrwBhTtUvIe4ijrBUO02p+jNlZ6F2kDeNvHl1lC6dTxaRBDtgF2SACU5BCVyCCqgCDB7BM3gHH9qD9qS9am/jaEKbvNkGv0b7+ga0hJ42</latexit><latexit sha1_base64="nymKrHjdrn0N2LZuD3dhXJghRd4=">AAACIHicbZBbSwJBHMVn7WZ22+qxlyEJlFB2u/sgSBL0aJEX0G2ZHWd1cPbCzGwh4pepL1MQRPVUn6ZxVajs/zK/OecMzPk7IaNCGsanlpibX1hcSi6nVlbX1jf0za2aCCKOSRUHLOANBwnCqE+qkkpGGiEnyHMYqTu98siv3xEuaODfyH5ILA91fOpSjKSSbL1Ytg9v91uYcgxjhLGSg61r2ulKxHlwDy8ysZWdnsUx5LK2njbyRjxwBMeFkwI0p8oU0mAyFVt/abUDHHnEl5ghIZqmEUprgLikmJFhqhUJEiLcQx3SVOgjjwhrENccwj034FB2CYzvP7MD5AnR9xyV8ZDsir/eSPzPa0bSPbMG1A8jSXysIspzIwZlAEfbgm3KCZasrwBhTtUvIe4ijrBUO02p+jNlZ6F2kDeNvHl1lC6dTxaRBDtgF2SACU5BCVyCCqgCDB7BM3gHH9qD9qS9am/jaEKbvNkGv0b7+ga0hJ42</latexit><latexit sha1_base64="nymKrHjdrn0N2LZuD3dhXJghRd4=">AAACIHicbZBbSwJBHMVn7WZ22+qxlyEJlFB2u/sgSBL0aJEX0G2ZHWd1cPbCzGwh4pepL1MQRPVUn6ZxVajs/zK/OecMzPk7IaNCGsanlpibX1hcSi6nVlbX1jf0za2aCCKOSRUHLOANBwnCqE+qkkpGGiEnyHMYqTu98siv3xEuaODfyH5ILA91fOpSjKSSbL1Ytg9v91uYcgxjhLGSg61r2ulKxHlwDy8ysZWdnsUx5LK2njbyRjxwBMeFkwI0p8oU0mAyFVt/abUDHHnEl5ghIZqmEUprgLikmJFhqhUJEiLcQx3SVOgjjwhrENccwj034FB2CYzvP7MD5AnR9xyV8ZDsir/eSPzPa0bSPbMG1A8jSXysIspzIwZlAEfbgm3KCZasrwBhTtUvIe4ijrBUO02p+jNlZ6F2kDeNvHl1lC6dTxaRBDtgF2SACU5BCVyCCqgCDB7BM3gHH9qD9qS9am/jaEKbvNkGv0b7+ga0hJ42</latexit><latexit sha1_base64="nymKrHjdrn0N2LZuD3dhXJghRd4=">AAACIHicbZBbSwJBHMVn7WZ22+qxlyEJlFB2u/sgSBL0aJEX0G2ZHWd1cPbCzGwh4pepL1MQRPVUn6ZxVajs/zK/OecMzPk7IaNCGsanlpibX1hcSi6nVlbX1jf0za2aCCKOSRUHLOANBwnCqE+qkkpGGiEnyHMYqTu98siv3xEuaODfyH5ILA91fOpSjKSSbL1Ytg9v91uYcgxjhLGSg61r2ulKxHlwDy8ysZWdnsUx5LK2njbyRjxwBMeFkwI0p8oU0mAyFVt/abUDHHnEl5ghIZqmEUprgLikmJFhqhUJEiLcQx3SVOgjjwhrENccwj034FB2CYzvP7MD5AnR9xyV8ZDsir/eSPzPa0bSPbMG1A8jSXysIspzIwZlAEfbgm3KCZasrwBhTtUvIe4ijrBUO02p+jNlZ6F2kDeNvHl1lC6dTxaRBDtgF2SACU5BCVyCCqgCDB7BM3gHH9qD9qS9am/jaEKbvNkGv0b7+ga0hJ42</latexit>
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Reducible and irreducible representations
W (E) =
0
@1 0 00 1 00 0 1
1
A
W (C+3 ) =
0
B@� 1
2 �p34 �
p34p
32
14 � 3
4p32 � 3
414
1
CA
W (C�3 ) =
0
B@� 1
2
p34
p34
�p32
14 � 3
4
�p32 � 3
414
1
CA
W (�d1) =
0
@1 0 00 0 �10 �1 0
1
A
W (�d2) =
0
B@� 1
2 �p34 �
p34
�p32
34 � 1
4
�p32 � 1
434
1
CA
The representation W decomposes into the two irreducible
representations A1 and E
W (x, y, z) = A1(y � z) + E(2x, y + z)
W (�d3) =
0
B@� 1
2
p34
p34p
32
34 � 1
4p32 � 1
434
1
CA
27
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Equivalent representations
T2(g) = AT2(g)A�1 8g 2 G
In our case, with
A =
0
@1 0 00 1 10 1 �1
1
A
W (g) = AV (g)A�1 8g 2 C3v
Thus W(g) is just V(g) in the coordinate system (x0, y
0, z
0)
Two representations and of a group G are said to be equivalent if there is a non-singular matrix A such that
T1 T2
This can be interpreted as a change of coordinates, with , i.e. u0 = Au x
0 = x, y
0 = y + z, z
0 = y � z
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Symmetry-adapted coordinatesThe block structure of the matrices of a reducible representation becomes apparent only when symmetry-adapted coordinates are used.
are symmetry-adapted coordinates for the vector representation of . C3v
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29
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APPENDIX: GroupsIn mathematics, a group G is a set of elements with the following properties
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2) There is a unique unit element e such that e � g = g � e = g, 8g 2 G
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g � g�1 = g�1 � g = e, 8g 2 G<latexit sha1_base64="OrY52KEiY5T3Az//S4vDNfer0lw=">AAACEnicbVDLSgMxFM3UV62vqks3wSK40DojvooUii50WcE+oFNLJr0zDc08SDJCGfoX+jO6EnXXH/BvTF+g1rPJyTnnkpzrRJxJZZpfRmpufmFxKb2cWVldW9/Ibm5VZRgLChUa8lDUHSKBswAqiikO9UgA8R0ONad7PfRrjyAkC4N71Yug6RMvYC6jRGmplT3ybMoExd5Dcmj1i+NjIhXhANuXthsKwjn2bBbgm1Y2Z+bNEfCQnBbOCtiaKlOSQxOUW9mB3Q5p7EOgKCdSNiwzUs2ECMUoh37GjiVEhHaJBw1NA+KDbCajYn28p9/GqgN4dP+ZTYgvZc93dMYnqiP/ekPxP68RK/eimbAgihUEVEe058YcqxAP94PbTABVvKcJoYLpX2LaIYJQpbeY0fVnys6S6nHeMvPW3UmudDVZRBrtoF20jyx0jkroFpVRBVH0jF7RB/o0nowX4814H0dTxmRmG/2CMfgGyVmcMQ==</latexit><latexit sha1_base64="OrY52KEiY5T3Az//S4vDNfer0lw=">AAACEnicbVDLSgMxFM3UV62vqks3wSK40DojvooUii50WcE+oFNLJr0zDc08SDJCGfoX+jO6EnXXH/BvTF+g1rPJyTnnkpzrRJxJZZpfRmpufmFxKb2cWVldW9/Ibm5VZRgLChUa8lDUHSKBswAqiikO9UgA8R0ONad7PfRrjyAkC4N71Yug6RMvYC6jRGmplT3ybMoExd5Dcmj1i+NjIhXhANuXthsKwjn2bBbgm1Y2Z+bNEfCQnBbOCtiaKlOSQxOUW9mB3Q5p7EOgKCdSNiwzUs2ECMUoh37GjiVEhHaJBw1NA+KDbCajYn28p9/GqgN4dP+ZTYgvZc93dMYnqiP/ekPxP68RK/eimbAgihUEVEe058YcqxAP94PbTABVvKcJoYLpX2LaIYJQpbeY0fVnys6S6nHeMvPW3UmudDVZRBrtoF20jyx0jkroFpVRBVH0jF7RB/o0nowX4814H0dTxmRmG/2CMfgGyVmcMQ==</latexit><latexit sha1_base64="OrY52KEiY5T3Az//S4vDNfer0lw=">AAACEnicbVDLSgMxFM3UV62vqks3wSK40DojvooUii50WcE+oFNLJr0zDc08SDJCGfoX+jO6EnXXH/BvTF+g1rPJyTnnkpzrRJxJZZpfRmpufmFxKb2cWVldW9/Ibm5VZRgLChUa8lDUHSKBswAqiikO9UgA8R0ONad7PfRrjyAkC4N71Yug6RMvYC6jRGmplT3ybMoExd5Dcmj1i+NjIhXhANuXthsKwjn2bBbgm1Y2Z+bNEfCQnBbOCtiaKlOSQxOUW9mB3Q5p7EOgKCdSNiwzUs2ECMUoh37GjiVEhHaJBw1NA+KDbCajYn28p9/GqgN4dP+ZTYgvZc93dMYnqiP/ekPxP68RK/eimbAgihUEVEe058YcqxAP94PbTABVvKcJoYLpX2LaIYJQpbeY0fVnys6S6nHeMvPW3UmudDVZRBrtoF20jyx0jkroFpVRBVH0jF7RB/o0nowX4814H0dTxmRmG/2CMfgGyVmcMQ==</latexit><latexit sha1_base64="OrY52KEiY5T3Az//S4vDNfer0lw=">AAACEnicbVDLSgMxFM3UV62vqks3wSK40DojvooUii50WcE+oFNLJr0zDc08SDJCGfoX+jO6EnXXH/BvTF+g1rPJyTnnkpzrRJxJZZpfRmpufmFxKb2cWVldW9/Ibm5VZRgLChUa8lDUHSKBswAqiikO9UgA8R0ONad7PfRrjyAkC4N71Yug6RMvYC6jRGmplT3ybMoExd5Dcmj1i+NjIhXhANuXthsKwjn2bBbgm1Y2Z+bNEfCQnBbOCtiaKlOSQxOUW9mB3Q5p7EOgKCdSNiwzUs2ECMUoh37GjiVEhHaJBw1NA+KDbCajYn28p9/GqgN4dP+ZTYgvZc93dMYnqiP/ekPxP68RK/eimbAgihUEVEe058YcqxAP94PbTABVvKcJoYLpX2LaIYJQpbeY0fVnys6S6nHeMvPW3UmudDVZRBrtoF20jyx0jkroFpVRBVH0jF7RB/o0nowX4814H0dTxmRmG/2CMfgGyVmcMQ==</latexit>
3) For each element g there is a unique inverse g�1<latexit sha1_base64="q+gKnL28hWJVlYeZH/t4l6/uI/g=">AAAB53icbVDLTgJBEOzFF+IL9ehlIjHxItk1vrgRvXjERB4JrGR26IWR2UdmZk3Ihm/Qk1Fv/o8/4N84i5CoWKfqquqkq71YcKVt+9PKLSwuLa/kVwtr6xubW8XtnYaKEsmwziIRyZZHFQoeYl1zLbAVS6SBJ7DpDa8yv/mAUvEovNWjGN2A9kPuc0a1kRr9u/TIGXeLJbtsT0Ayclo5qxBnpsxICaaodYsfnV7EkgBDzQRVqu3YsXZTKjVnAseFTqIwpmxI+9g2NKQBKjedXDsmB34kiR4gmcw/sykNlBoFnskEVA/UXy8T//PaifYv3JSHcaIxZCZiPD8RREckK016XCLTYmQIZZKbKwkbUEmZNq8pmPpzZedJ47js2GXn5qRUvZw+Ig97sA+H4MA5VOEaalAHBvfwBK/wZnHr0Xq2Xr6jOWu6swu/YL1/ATUhjIQ=</latexit><latexit sha1_base64="q+gKnL28hWJVlYeZH/t4l6/uI/g=">AAAB53icbVDLTgJBEOzFF+IL9ehlIjHxItk1vrgRvXjERB4JrGR26IWR2UdmZk3Ihm/Qk1Fv/o8/4N84i5CoWKfqquqkq71YcKVt+9PKLSwuLa/kVwtr6xubW8XtnYaKEsmwziIRyZZHFQoeYl1zLbAVS6SBJ7DpDa8yv/mAUvEovNWjGN2A9kPuc0a1kRr9u/TIGXeLJbtsT0Ayclo5qxBnpsxICaaodYsfnV7EkgBDzQRVqu3YsXZTKjVnAseFTqIwpmxI+9g2NKQBKjedXDsmB34kiR4gmcw/sykNlBoFnskEVA/UXy8T//PaifYv3JSHcaIxZCZiPD8RREckK016XCLTYmQIZZKbKwkbUEmZNq8pmPpzZedJ47js2GXn5qRUvZw+Ig97sA+H4MA5VOEaalAHBvfwBK/wZnHr0Xq2Xr6jOWu6swu/YL1/ATUhjIQ=</latexit><latexit sha1_base64="q+gKnL28hWJVlYeZH/t4l6/uI/g=">AAAB53icbVDLTgJBEOzFF+IL9ehlIjHxItk1vrgRvXjERB4JrGR26IWR2UdmZk3Ihm/Qk1Fv/o8/4N84i5CoWKfqquqkq71YcKVt+9PKLSwuLa/kVwtr6xubW8XtnYaKEsmwziIRyZZHFQoeYl1zLbAVS6SBJ7DpDa8yv/mAUvEovNWjGN2A9kPuc0a1kRr9u/TIGXeLJbtsT0Ayclo5qxBnpsxICaaodYsfnV7EkgBDzQRVqu3YsXZTKjVnAseFTqIwpmxI+9g2NKQBKjedXDsmB34kiR4gmcw/sykNlBoFnskEVA/UXy8T//PaifYv3JSHcaIxZCZiPD8RREckK016XCLTYmQIZZKbKwkbUEmZNq8pmPpzZedJ47js2GXn5qRUvZw+Ig97sA+H4MA5VOEaalAHBvfwBK/wZnHr0Xq2Xr6jOWu6swu/YL1/ATUhjIQ=</latexit><latexit sha1_base64="q+gKnL28hWJVlYeZH/t4l6/uI/g=">AAAB53icbVDLTgJBEOzFF+IL9ehlIjHxItk1vrgRvXjERB4JrGR26IWR2UdmZk3Ihm/Qk1Fv/o8/4N84i5CoWKfqquqkq71YcKVt+9PKLSwuLa/kVwtr6xubW8XtnYaKEsmwziIRyZZHFQoeYl1zLbAVS6SBJ7DpDa8yv/mAUvEovNWjGN2A9kPuc0a1kRr9u/TIGXeLJbtsT0Ayclo5qxBnpsxICaaodYsfnV7EkgBDzQRVqu3YsXZTKjVnAseFTqIwpmxI+9g2NKQBKjedXDsmB34kiR4gmcw/sykNlBoFnskEVA/UXy8T//PaifYv3JSHcaIxZCZiPD8RREckK016XCLTYmQIZZKbKwkbUEmZNq8pmPpzZedJ47js2GXn5qRUvZw+Ig97sA+H4MA5VOEaalAHBvfwBK/wZnHr0Xq2Xr6jOWu6swu/YL1/ATUhjIQ=</latexit>
1) There is a composition law that asigns an element to each ordered pair of elementsg3
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g1 � g2 = g3<latexit sha1_base64="DkYO4LuiTbZJXrXabBeRzpPDXlQ=">AAAB83icbVDLSgNBEOz1GeNr1aOXwSB4CrvxmYMQ9OIxgnlAEpbZSWczZPbBzGwghHyJnkS9+Sf+gH/jbExAjXWqrqqG7vITwZV2nE9raXlldW09t5Hf3Nre2bX39usqTiXDGotFLJs+VSh4hDXNtcBmIpGGvsCGP7jN/MYQpeJx9KBHCXZCGkS8xxnVRvJsO/DcNuOSkcArXQfeqWcXnKIzBcnIefmiTNy5MicFmKHq2R/tbszSECPNBFWq5TqJ7oyp1JwJnOTbqcKEsgENsGVoRENUnfH08gk57sWS6D6S6fwzO6ahUqPQN5mQ6r7662Xif14r1b2rzphHSaoxYiZivF4qiI5JVgDpcolMi5EhlEluriSsTyVl2tSUN+8vPLtI6qWi6xTd+7NC5WZWRA4O4QhOwIVLqMAdVKEGDIbwBK/wZqXWo/VsvXxHl6zZzgH8gvX+BfYIkEA=</latexit><latexit sha1_base64="DkYO4LuiTbZJXrXabBeRzpPDXlQ=">AAAB83icbVDLSgNBEOz1GeNr1aOXwSB4CrvxmYMQ9OIxgnlAEpbZSWczZPbBzGwghHyJnkS9+Sf+gH/jbExAjXWqrqqG7vITwZV2nE9raXlldW09t5Hf3Nre2bX39usqTiXDGotFLJs+VSh4hDXNtcBmIpGGvsCGP7jN/MYQpeJx9KBHCXZCGkS8xxnVRvJsO/DcNuOSkcArXQfeqWcXnKIzBcnIefmiTNy5MicFmKHq2R/tbszSECPNBFWq5TqJ7oyp1JwJnOTbqcKEsgENsGVoRENUnfH08gk57sWS6D6S6fwzO6ahUqPQN5mQ6r7662Xif14r1b2rzphHSaoxYiZivF4qiI5JVgDpcolMi5EhlEluriSsTyVl2tSUN+8vPLtI6qWi6xTd+7NC5WZWRA4O4QhOwIVLqMAdVKEGDIbwBK/wZqXWo/VsvXxHl6zZzgH8gvX+BfYIkEA=</latexit><latexit sha1_base64="DkYO4LuiTbZJXrXabBeRzpPDXlQ=">AAAB83icbVDLSgNBEOz1GeNr1aOXwSB4CrvxmYMQ9OIxgnlAEpbZSWczZPbBzGwghHyJnkS9+Sf+gH/jbExAjXWqrqqG7vITwZV2nE9raXlldW09t5Hf3Nre2bX39usqTiXDGotFLJs+VSh4hDXNtcBmIpGGvsCGP7jN/MYQpeJx9KBHCXZCGkS8xxnVRvJsO/DcNuOSkcArXQfeqWcXnKIzBcnIefmiTNy5MicFmKHq2R/tbszSECPNBFWq5TqJ7oyp1JwJnOTbqcKEsgENsGVoRENUnfH08gk57sWS6D6S6fwzO6ahUqPQN5mQ6r7662Xif14r1b2rzphHSaoxYiZivF4qiI5JVgDpcolMi5EhlEluriSsTyVl2tSUN+8vPLtI6qWi6xTd+7NC5WZWRA4O4QhOwIVLqMAdVKEGDIbwBK/wZqXWo/VsvXxHl6zZzgH8gvX+BfYIkEA=</latexit><latexit sha1_base64="DkYO4LuiTbZJXrXabBeRzpPDXlQ=">AAAB83icbVDLSgNBEOz1GeNr1aOXwSB4CrvxmYMQ9OIxgnlAEpbZSWczZPbBzGwghHyJnkS9+Sf+gH/jbExAjXWqrqqG7vITwZV2nE9raXlldW09t5Hf3Nre2bX39usqTiXDGotFLJs+VSh4hDXNtcBmIpGGvsCGP7jN/MYQpeJx9KBHCXZCGkS8xxnVRvJsO/DcNuOSkcArXQfeqWcXnKIzBcnIefmiTNy5MicFmKHq2R/tbszSECPNBFWq5TqJ7oyp1JwJnOTbqcKEsgENsGVoRENUnfH08gk57sWS6D6S6fwzO6ahUqPQN5mQ6r7662Xif14r1b2rzphHSaoxYiZivF4qiI5JVgDpcolMi5EhlEluriSsTyVl2tSUN+8vPLtI6qWi6xTd+7NC5WZWRA4O4QhOwIVLqMAdVKEGDIbwBK/wZqXWo/VsvXxHl6zZzgH8gvX+BfYIkEA=</latexit>
4) The composition law is associative(g1 � g2) � g3 = g1 � (g2 � g3)
<latexit sha1_base64="nwGBcQTcKfIj4Hw45RRZaY0BlZ0=">AAACE3icbVBLTwIxGPwWX4ivVY9eGokJXMguPjmYEL14xETABMimW7rQ0H2k7ZqQDT9D/4yejHrSH+C/sYtAVJzTdGaafDNuxJlUlvVpZBYWl5ZXsqu5tfWNzS1ze6chw1gQWichD8WtiyXlLKB1xRSnt5Gg2Hc5bbqDy9Rv3lEhWRjcqGFEOz7uBcxjBCstOaZV6Dl2mzBBUM8pF6fs8BzNdJ0oz/SiY+atkjUGSslx5aSC7KkyJXmYoOaY7+1uSGKfBopwLGXLtiLVSbBQjHA6yrVjSSNMBrhHW5oG2Keyk4ybjdCBFwqk+hSN3z+zCfalHPquzvhY9eVfLxX/81qx8s46CQuiWNGA6Ij2vJgjFaJ0INRlghLFh5pgIpi+EpE+FpgoPWNO158rO08a5ZJtlezro3z1YjJEFvZgHwpgwylU4QpqUAcCD/AEr/Bm3BuPxrPx8h3NGJM/u/ALxscXzLmbgQ==</latexit><latexit sha1_base64="nwGBcQTcKfIj4Hw45RRZaY0BlZ0=">AAACE3icbVBLTwIxGPwWX4ivVY9eGokJXMguPjmYEL14xETABMimW7rQ0H2k7ZqQDT9D/4yejHrSH+C/sYtAVJzTdGaafDNuxJlUlvVpZBYWl5ZXsqu5tfWNzS1ze6chw1gQWichD8WtiyXlLKB1xRSnt5Gg2Hc5bbqDy9Rv3lEhWRjcqGFEOz7uBcxjBCstOaZV6Dl2mzBBUM8pF6fs8BzNdJ0oz/SiY+atkjUGSslx5aSC7KkyJXmYoOaY7+1uSGKfBopwLGXLtiLVSbBQjHA6yrVjSSNMBrhHW5oG2Keyk4ybjdCBFwqk+hSN3z+zCfalHPquzvhY9eVfLxX/81qx8s46CQuiWNGA6Ij2vJgjFaJ0INRlghLFh5pgIpi+EpE+FpgoPWNO158rO08a5ZJtlezro3z1YjJEFvZgHwpgwylU4QpqUAcCD/AEr/Bm3BuPxrPx8h3NGJM/u/ALxscXzLmbgQ==</latexit><latexit sha1_base64="nwGBcQTcKfIj4Hw45RRZaY0BlZ0=">AAACE3icbVBLTwIxGPwWX4ivVY9eGokJXMguPjmYEL14xETABMimW7rQ0H2k7ZqQDT9D/4yejHrSH+C/sYtAVJzTdGaafDNuxJlUlvVpZBYWl5ZXsqu5tfWNzS1ze6chw1gQWichD8WtiyXlLKB1xRSnt5Gg2Hc5bbqDy9Rv3lEhWRjcqGFEOz7uBcxjBCstOaZV6Dl2mzBBUM8pF6fs8BzNdJ0oz/SiY+atkjUGSslx5aSC7KkyJXmYoOaY7+1uSGKfBopwLGXLtiLVSbBQjHA6yrVjSSNMBrhHW5oG2Keyk4ybjdCBFwqk+hSN3z+zCfalHPquzvhY9eVfLxX/81qx8s46CQuiWNGA6Ij2vJgjFaJ0INRlghLFh5pgIpi+EpE+FpgoPWNO158rO08a5ZJtlezro3z1YjJEFvZgHwpgwylU4QpqUAcCD/AEr/Bm3BuPxrPx8h3NGJM/u/ALxscXzLmbgQ==</latexit><latexit sha1_base64="nwGBcQTcKfIj4Hw45RRZaY0BlZ0=">AAACE3icbVBLTwIxGPwWX4ivVY9eGokJXMguPjmYEL14xETABMimW7rQ0H2k7ZqQDT9D/4yejHrSH+C/sYtAVJzTdGaafDNuxJlUlvVpZBYWl5ZXsqu5tfWNzS1ze6chw1gQWichD8WtiyXlLKB1xRSnt5Gg2Hc5bbqDy9Rv3lEhWRjcqGFEOz7uBcxjBCstOaZV6Dl2mzBBUM8pF6fs8BzNdJ0oz/SiY+atkjUGSslx5aSC7KkyJXmYoOaY7+1uSGKfBopwLGXLtiLVSbBQjHA6yrVjSSNMBrhHW5oG2Keyk4ybjdCBFwqk+hSN3z+zCfalHPquzvhY9eVfLxX/81qx8s46CQuiWNGA6Ij2vJgjFaJ0INRlghLFh5pgIpi+EpE+FpgoPWNO158rO08a5ZJtlezro3z1YjJEFvZgHwpgwylU4QpqUAcCD/AEr/Bm3BuPxrPx8h3NGJM/u/ALxscXzLmbgQ==</latexit>
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