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Research ArticleDistortion Type Theorems for Functions inthe Logarithmic Bloch Space
Armando J. GarcΓa-OrtΓz,1 Milton del Castillo Lesmes Acosta,2
and Julio C. Ramos-FernΓ‘ndez3
1 AΜrea de MatemaΜtica, Universidad Nacional Experimental de Guayana, Pto. Ordaz, BolΜΔ±var, Venezuela2Proyecto Curricular de MatemaΜticas, Facultad de Ciencias y EducacioΜn, Universidad Distrital Francisco JoseΜ de Caldas,Carrera 3 No. 26 A-40, BogotaΜ, Colombia3Departamento de MatemaΜtica, Universidad de Oriente, 6101 CumanaΜ, Sucre, Venezuela
Correspondence should be addressed to Julio C. Ramos-FernaΜndez; [email protected]
Received 23 January 2017; Revised 22 March 2017; Accepted 2 April 2017; Published 16 April 2017
Academic Editor: John R. Akeroyd
Copyright Β© 2017 Armando J. GarcΜΔ±a-OrtΜΔ±z et al.This is an open access article distributed under theCreativeCommonsAttributionLicense, which permits unrestricted use, distribution, and reproduction in anymedium, provided the originalwork is properly cited.
We establish distortion type theorems for locally schlicht functions and for functions having branch points satisfying a normalizedBloch condition in the closed unit ball of the logarithmic Bloch spaceBlog. As a consequence of our results we have estimations ofthe schlicht radius for functions in these classes.
1. Introduction
One of the most important results in the area of geometrictheory of functions of a complex variable is the celebrateddistortionβs theorem established by Koebe and Bieberbach[1, 2] at the beginning of the twentieth century. Koebe andBieberbach showed that the range of any function π in theclass S of all conformal functions on D, the open unit diskof the complex plane C, normalized such that π(0) = 0 =π(0) β 1 contain the Euclidean disk with center at the originand radius 1/4. This last result is today known as Koebe 1/4Theorem and, in particular, shows that Blochβs constant (see[3]) is greater than or equal to 1/4. Koebe and Bieberbachfound sharp lower and upper bounds for the growth and thedistortion of conformal maps in the class S; more precisely,they showed that for any π β S and π§ β D the followingestimations hold.
(1) Growth theorem:|π§|
(1 + |π§|)2 β€ π (π§) β€|π§|
(1 β |π§|)2 (1)(2) Distortion theorem:
1 β |π§|(1 + |π§|)3 β€
π (π§) β€ 1 + |π§|(1 β |π§|)3 (2)
with equality if and only if π is a rotation of the Koebefunction defined by
πΎ (π§) = π§(1 β π§)2 , (π§ β D) , (3)which also belongs to the classS. In particular, the distortiontheorem implies that the class S is contained in the closedball with center at the origin and radius 8 of π-Bloch spaceBπ for all π β₯ 3 (see Section 3 for the definition ofBπ). Formore properties of conformal maps and distortion theorem,we recommend the excellent books [4, 5].
Although the distortion theorem gives sharp bounds forthe modulus of the derivative of functions in the class S,it cannot be applied to the bigger class of locally schlichtfunctions defined on D satisfying the normalized Blochconditions π(0) = 0 = π(0) β 1 (recall that a holomorphicfunction π is locally schlicht on D if π(π§) ΜΈ= 0 for all π§ βD). Many authors have obtained distortion type theorems orlower bounds for the modulus or real part of the derivative oflocally schlicht functions in Bloch-type spaces. The pioneerwork about this subject appears in 1992 and is due to Liu andMinda [6]. They established distortion theorems for locallyschlicht functions π in the classical Bloch spaceB satisfyingthe conditions π(0) = 0, π(0) = 1, and βπβB = 1 (seeSection 3 for the definition of Bloch space). Liu and Minda
HindawiJournal of Function SpacesVolume 2017, Article ID 8694516, 10 pageshttps://doi.org/10.1155/2017/8694516
https://doi.org/10.1155/2017/8694516
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2 Journal of Function Spaces
give sharp lower bounds for |π(π§)| and for Reπ(π§) and asconsequence of their results they obtain a lower bound forBlochβs constant.Determination of the (locally schlicht) Blochconstant is still an open problem. By Landauβs reduction, itis enough to consider those functions with Bloch seminormnot greater than 1. Hence, it is important to consider certainsubclasses of functions in Bloch spaces having seminorm notgreater than 1.
The results of Liu and Minda in [6] have been extendedto other classes of locally schlicht functions or to functionshaving branch points in the Bloch space by Yanagihara [7],Bonk et al. [8, 9], and Graham andMinda [10].The extensionof the above results to π-Bloch spaces was obtained by Teradaand Yanagihara [11] and by Zheng and Wang [12]. It is anopen problem to obtain distortion type theorems for locallyschlicht functions in other spaces of analytic functions.
In this article we extend the results of Liu and Minda[6] to the logarithmic Bloch space Blog which we define inSection 3; we obtain lower bounds for the modulus and thereal part of the derivative of locally schlicht functions and forfunctions having branch points in the closed unit ball ofBlogsatisfying a normalized Bloch conditionπ(0) = 0 = π(0)β1.Our resultswill be showed in Sections 4 and 5, as consequenceof our results, in Section 6, we obtain lower bounds for theschlicht radius of functions in these classes.
2. Some Preliminaries: Juliaβs Lemma
In this section we gather some notations, definitions, andresults that we will need through this note. We denote by Dthe open unit disk in the complex plane C, with center atthe origin and radius 1; πD denotes the boundary of D. Thespace of all complex and holomorphic functions on D, as isusual, is denoted byπ»(D). A function π β π»(D) is said to benormalized if π(0) = 0 and π(0) = 1 and π is locally schlichtor locally univalent if π(π§) ΜΈ= 0 for all π§ β D. A point π§0 is abranch point for π if π(π§0) = 0. For π > 0, we define
Ξ (1, π) = {π§ β D : |1 β π§|21 β |π§|2 < π} . (4)Ξ(1, π) is known as a horodisk D; that is, it is an Euclideandisk contained inDwhich is tangent to πD at 1. Furthermore,Ξ(1, π) has center at 1/(1+π) and radius π/(1+π). The closureof Ξ(1, π) relative toD is denoted by Ξ(1, π). Observe that 1 βΞ(1, π) but (1 β π)/(1 + π) β Ξ(1, π). With these notations, wecan enunciate the well known Juliaβs Lemma; the reader canconsult the excellent book of Ahlfors [13] for its proof.
Lemma 1 (Juliaβs Lemma). Suppose that π€ is a complex andholomorphic function onDβͺ{1} such thatπ€mapsD intoH+ ={π§ β C : Re(π§) > 0}, the right half-plane, and π€(1) = 0. Then,for any π > 0, the functionπ€maps the horodisk Ξ(1, π) into theEuclidean disk {π§ β C : |π§ β ππ| < ππ}, where π = βπ€(1) > 0.Furthermore, a boundary point of the first disk is mapped onthe boundary of the second disk if and only if π€ is a conformalfunction mapping D onto H+ and satisfying π€(1) = 0.
In 1992, Liu andMinda [6] established distortion theoremfor functions in the Bloch space; they showed the followingresults which are consequences of Juliaβs Lemma. We includethe proof of the first one to illustrate the application of JuliaβsLemma.
Lemma2 ([6, corollary in Section 1]). Letπ€ be a holomorphicfunction onDβͺ{1}. Suppose thatπ€mapsD into the right half-plane H+ and that π€(1) = 0. Then π = βπ€(1) > 0 and
Reπ€ (π₯) β€ 2π1 β π₯1 + π₯ , (5)for all π₯ β (β1, 1), with equality for some π₯ β (β1, 1) if andonly if
π€ (π§) = 2π1 β π§1 + π§ , (6)for all π§ β D.Proof. Indeed, let us fix π₯ β (β1, 1); then π = (1βπ₯)/(1+π₯) >0 and by Juliaβs Lemma, π€ maps Ξ(1, π) into the Euclideandisk π·(ππ, ππ). In particular, since π₯ β Ξ(1, π), then π€(π₯) βπ·(ππ, ππ); this fact implies that Reπ€(π₯) β€ 2ππ. Furthermore,π₯ β πΞ(1, π); hence if Reπ€(π₯) = 2ππ, then we conclude thatπ€(π₯) = 2ππ β ππ·(ππ, ππ) and, by Juliaβs Lemma, this last factoccurs if π€ is the conformal map from D onto H+ such thatπ€(1) = 0; that is,π€(π§) = 2π((1βπ§)/(1+π§)) for all π§ β D. Thisshows the lemma.
Lemma 3 ([6, corollary to Theorem 3]). Let π be a holomor-phic function onDβͺ{1}. Suppose thatπ(D) β D,π(1) = 1 andthat all the zeros of π have multiplicity at least π. If π(1) = π,then
(1) |π(π₯)| β₯ π₯π for all π₯ β [0, 1), with equality for someπ₯ β [0, 1) if and only if π(π§) = π§π for all π§ β D;(2) Re(π(π₯)) β₯ π₯π for all (π β 1)/(π + 1) β€ π₯ < 1, with
equality for some π₯ β [(π β 1)/(π + 1), 1) if and only ifπ(π§) = π§π for all π§ β D.We finish this section by establishing the following ele-
mentary property of the complex exponential. We thank thereviewer for providing us the following simple demonstrationof this fact.
Lemma 4. Let π₯ β [0, 1) be fixed and π·π₯ the Euclidean diskwith center at (1 β π₯)/(1 + π₯) and radius (1 β π₯)/(1 + π₯); then
min {Re (exp (βπ§)) : π§ β π·π₯} = exp (β21 β π₯1 + π₯) . (7)Proof. Let π = (1 β π₯)/(1 + π₯) for simplicity and let π(π§) =ππ(π§β1). Since 1 + π§π(π§)/π(π§) = 1 β ππ§ has positive real parton |π§| < 1, the functionπ is convex. In particular, Re(π(π§)) >π(β1) = πβ2π, which proves the assertion.3. Logarithmic Bloch Space
In this section we gather the definition and some of theproperties of the logarithmic Bloch space Blog. Let us recall
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that a function weight π on D is a bounded, positive, andcontinuous function defined on D. Given a weight π on D,π-Bloch space, denoted by Bπ, consists of all holomorphicfunctions π on D such that
ππ fl supπ§βD
π (π§) π (π§) < β. (8)It is known that if the weight π is radial, that is, π(π§) = π(|π§|)for all π§ β D, then Bπ is a Banach space with the normβπβBπ = |π(0)| + βπβπ. When π(π§) = 1β |π§|2, with π§ β D,Bπbecomes the Bloch space which is denoted byB, while whenπ(π§) = (1 β |π§|2)π, with π§ β D and π > 0 fixed, we obtainπ-Bloch space which is denoted byBπ.
Clearly, the function πlog, defined byπlog (π§) = [log( π1 β |π§|2)]
β1 , (9)defines a weight on D. Hence, the space Blog = Bπlog is aBanach space with the norm
πBlog = π (0) + πlogfl π (0) + sup
π§βD
π (π§)log (π/ (1 β |π§|2)) .
(10)
We callBlog as the logarithmic Bloch space. In the next resultwe are going to show that Blog is a subspace of Bπ for allπ β₯ 1.Proposition 5. The spaceBlog is contained inBπ, for all π β₯1. Furthermore, πBπ β€ πBlog , (11)for all function π β Blog.Proof. It is enough to show that for π β₯ 1 fixed
(1 β |π§|2)π log( π1 β |π§|2) β€ 1, (12)for all π§ β D. But, this last inequality is true since the function
β (π‘) = π‘π log(ππ‘ ) β 1, (13)with π‘ β (0, 1], is increasing and β(1) = 0.
Also, we have the following very useful identity (seeLemma 3.3 in [12]).
Lemma 6. If π β Blog, π(0) = 0, π(0) = 1, and βπβlog β€ 1,then π(0) = 0.Proof. Suppose that π β Blog, π(0) = 0, π(0) = 1, andβπβlog β€ 1. Then, for each π§ β D, we have
π (π§) β€ log( π1 β |π§|2) . (14)
Taylorβs theorem implies that
1 + π (0) π§ + π (|π§|)2 β€ log2 ( π1 β |π§|2)= (1 + |π§|2 + π (|π§|))2 ,
(15)
as π§ β 0. But since1 + π (0) π§ + π (|π§|)2
= 1 + 2Re (π (0) π§) + π (|π§|) , (16)as π§ β 0, and
(1 + |π§|2 + π (|π§|))2 = 1 + π (|π§|) , (17)as π§ β 0, we obtain from (15) that
2Re (π (0) π§) β€ π (|π§|) , (18)as π§ β 0. Now, if we consider π§ = ππ(0)/|π(0)| with π > 0small in (18), we conclude |π(0)| = 0 and we are done.
The following functions play a very important role in ourwork; theywill be used to get lower bounds for locally schlichtfunctions and for functions having branch points in certainclasses in the logarithmic Bloch space. From now, we uselog(π€) to denote the principal logarithmic of the complexnumber π€ ΜΈ= 0. Observe that the principal logarithmic isa holomorphic function on π·(1, 1), the Euclidean disk withcenter at 1 and radius 1:
(1) For each π β N, we setπΉπ (π§) = β«π§
0(1 β π /ππ1 β πππ )
π (1 β 2 log (1 β πππ )) ππ , (19)where ππ = βπ/(π + 2) and π§ β D. Clearly, πΉπ β π»(D) for allπ β N, πΉπ(0) = 0, and πΉπ(0) = 1.
(2) For π§ β D, we defineπΉ (π§) = β«π§
0exp (β 2π 1 β π ) (1 β 2 log (1 β π )) ππ . (20)
We can see that πΉ β π»(D), πΉ(0) = 0, and πΉ(0) = 1.Also we have that the function πΉ satisfies the following
properties.
Proposition 7. The function πΉ belongs toBlog. Furthermore,supπ§βDπlog(π§)|πΉ(|π§|)| = 1 but βπΉβlog > 1.Proof. We see that βπΉβlog > 1. Indeed, we have
βπΉβlog β₯πΉ (π/2)1 β log (1 β |π/2|2)
= exp (2/5)1 β log (3/4)β(1 β log(54))2 + 4 arctan2 (12)
β 1.4014837 > 1.
(21)
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Now, we are going to show that πΉ β Blog. Since thefunction exp(β2π§/(1 β π§)) is holomorphic on D, then themodulus maximum principle tells us that its maximum valueis attained in the boundary πD. But if |π§| = 1 then |1 β π§|2 =2(1 β Re(π§)) and hence
supπ§βD
exp(β 2π§1 β π§) = sup|π§|=1 exp(2 β
2 (1 β Re (π§))|1 β π§|2 )
= π.(22)
On the other hand, for each π§ β D, we have |arg(1βπ§)| < π/2and
supπ§βD
arg (1 β π§)1 β log (1 β |π§|2) β€π2 . (23)
Furthermore, using elementary calculus, we can see that thereal functionπ»(π‘) = π2 β (1 β π‘)(1 + π‘)3 is nonnegative for allπ‘ β [0, 1] (its minimum value isπ»(1/2) = π2 β 27/16 β 5.70).Hence for any π‘ β [0, 1) we obtain
1 β 2 log (1 β π‘) β 3 (1 β log (1 β π‘2))= log((1 β π‘) (1 + π‘)3π2 ) β€ 0.
(24)
This last implies that
1 β 2 log (1 β π‘)1 β log (1 β π‘2) β€ 3, (25)for all π‘ β [0, 1). We conclude that for any π§ β D such that|1 β π§|2 β€ π
log (π/ |1 β π§|2)1 β log (1 β |π§|2) =1 β 2 log (|1 β π§|)1 β log (1 β |π§|2)
β€ 1 β 2 log (1 β |π§|)1 β log (1 β |π§|2) β€ 3,(26)
while for π§ β D such that |1 β π§|2 > π we havelog (π/ |1 β π§|2)1 β log (1 β |π§|2) =
log (|1 β π§|2 /π)1 β log (1 β |π§|2) β€ log (4) β 1. (27)
These last inequalities, (22) and (23), imply that
βπΉβlog β€ π (3 + log (4) β 1 + 2 (π2 ))= π (2 + log (4) + π)
(28)
which shows that πΉ β Blog.Now, we are going to show that supπ§βDπlog(π§)|πΉ(|π§|)| = 1.
Observe that πlog(0)|πΉ(0)| = 1. Also the real functionπ»(π‘) =exp(β2π‘/(1βπ‘))(1β2log(1βπ‘))β1+ log(1βπ‘2)with π‘ β [0, 1)
satisfies π»(0) = 0, π»(π‘) β ββ as π‘ β 1β and it is strictlydecreasing since
π» (π‘)= β 2π‘1 β π‘2
β 21 β π‘ ( 21 β π‘ log( π(1 β π‘)2) β 1) exp (β2π‘1 β π‘)
< 0,
(29)
for all π‘ β [0, 1). Hence we conclude that π»(π‘) β€ 0 for allπ‘ β [0, 1) which shows the affirmation.For the sequence {πΉπ}, we have the following properties.
Proposition 8. Functions πΉπ with π β N belong to Blog andsatisfy
limπββ
πΉπ (π§) = πΉ (π§) , (30)for each π§ β D. Furthermore, for each π β N βπΉπβlog > 1, infact, supπ§βDπlog(π§)|πΉπ(|π§|)| > 1.Proof. Clearly, for any π β N, the function πΉπ belongsto Blog since πΉπ β π»(D). We are going to show thatsupπ§βDπlog(π§)|πΉπ(|π§|)| > 1. It is enough to show that thereexists a π‘0 β (ππ, 1) such thatπ»(π‘0) > 0, where
π»(π‘) = (π‘/ππ β 11 β πππ‘ )π
log( π(1 β πππ‘)2)β log( π1 β π‘2 ) ,
(31)
with π‘ β (ππ, 1). Observe that, for π‘π = πππ with π = 2/(1 +π2π) > 1, we have π‘π β (ππ, 1), π β 1 = 1 β ππ2π , and 1 β π2π2π =(1 β ππ2π)2 which implies thatπ»(π‘π) = 0. Also,π» (π‘)
= π (π‘/ππ β 11 β πππ‘ )πβ1 1/ππ β ππ(1 β πππ‘)2 log(
π(1 β πππ‘)2)
+ (π‘/ππ β 11 β πππ‘ )π 2ππ1 β πππ‘ β
2π‘1 β π‘2 ;(32)
hence
π» (π‘π) = π 1/ππ β ππ(1 β ππ2π)2 log(π
(1 β ππ2π)2) +2ππ1 β ππ2π
β 2πππ1 β π2π2π =π
(1 β ππ2π)2 (1ππ β ππ)
β (log( π(1 β ππ2π)2) β1π
2π2π1 β π2π)
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= π(1 β ππ2π)2 (1ππ β ππ)
β (log( π(1 β ππ2π)2) β 1) ,(33)
since π β 1 = 1 β ππ2π , 1 β π2π2π = (1 β ππ2π)2, and π(π2π + 1) = 2and we have used that ππ = βπ/(π + 2) in the last equality.Thus, we conclude that π»(π‘π) > 0 and since π»(π‘π) = 0, thenthere exists π‘0 β (π‘π, 1) such that π»(π‘0) > 0. This shows theaffirmation. The other properties of πΉπβs are clear.4. A Distortion Theorem for Locally SchlichtFunctions inBlog
In this section we establish a distortion theorem for locallyschlicht functions in the closed unit ball of Blog satisfyingnormalized Bloch conditions. We denote by π½(β)log the class ofall holomorphic functions π β Blog such that π is locallyschlicht, π(0) = 0, π(0) = 1, and βπβlog β€ 1. With thesenotations, we have the following result.
Theorem 9. If π β π½(β)log then we have the following:(1) |π(π§)| β₯ πΉ(|π§|) = exp(β2|π§|/(1 β |π§|))(1 β 2 log(1 β|π§|)) for all π§ β D. There is not a function π0 β π½(β)log
such that |π0(π§0)| = πΉ(|π§0|) for some π§0 β D \ {0}.(2) Reπ(π§) β₯ πΉ(|π§|) = exp(β2|π§|/(1 β |π§|))(1 β 2log(1 β|π§|)) for all |π§| β€ 1/2. There is not a function π0 β π½(β)log
such thatReπ0(π§0) = πΉ(|π§0|) for some π§0 β π·(0, 1/2)\{0}.Proof. (1) Suppose that π β π½(β)log . Let us fix |π| = 1 and we setthe function
π (π’) = (1 β 2 log(1 + π’2 ))β1 π (1 β π’2 π) , (34)
with π’ β D, where log(π€) denotes the principal logarithmicof π€ β π·(1, 1). Clearly π is holomorphic on D \ {β1} andπ(1) = 1 because π(0) = 1. Since π is locally schlicht on D,we have that π(π§) ΜΈ= 0 for all π§ β D. Furthermore,
π (π’)= 21 + π’ (1 β 2 log(1 + π’2 ))
β2 π (1 β π’2 π)+ (1 β 2 log(1 + π’2 ))
β1 π (1 β π’2 π)(βπ2) .(35)
In particular, π(1) = 1 since π(0) = 1 and π(0) = 0 (byLemma 6). Also, for any π’ β D, we have
π (π’) = 1 β 2 log(1 + π’2 )β1 π (1 β π’2 π)
β€ 1Re (1 β 2 log ((1 + π’) /2))β log( π1 β |(1 β π’) /2|2) =
1log (π/ |(1 + π’) /2|2)
β log (π/1 β |(1 β π’) /2|2) < 1
(36)
and πmapsD intoD \ {0}. Hence, there exists a holomorphicfunction π€mapping the unit disk D into the right half-planeH+ = {π§ : Re(π§) > 0} and such that π(π’) = exp{βπ€(π’)}for all π’ β D. Observe that π€(1) = 0 since π(1) = 1 andπ = βπ€(1) = 1. Invoking Lemma 2, it follows that
Re (π€ (π₯)) β€ 21 β π₯1 + π₯ , (37)for all π₯ β (β1, 1). Henceπ (π₯) = exp {βπ€ (π₯)} = exp (βRe {π€ (π₯)})
β₯ exp(β21 β π₯1 + π₯) ,(38)
for all π₯ β (β1, 1). That is,(1 β 2 log(1 + π₯2 ))
β1 π (1 β π₯2 π)
β₯ exp(β21 β π₯1 + π₯) ,(39)
for all π₯ β (β1, 1).Making the change π = (1βπ₯)/2, we obtain that π β (0, 1)
and π (ππ) β₯ exp(β2 π1 β π) (1 β 2 log (1 β π)) . (40)Therefore, if we consider π = |π§| with π§ β D ΜΈ= 0 and we takeπ = π§/|π§|, we conclude that
π (π§) β₯ exp(β2 |π§|1 β |π§|) (1 β 2 log (1 β |π§|))= πΉ (|π§|) .
(41)
This shows inequality (1).Now, if there existsπ0 β π½(β)log such that |π0(π§0)| = πΉ(|π§0|)
for some π§0 β D\{0}, then arguing as in the proof of inequality(1), for π0 = π§0/|π§0|, the function,
π0 (π§) = (1 β 2 log(1 + π§2 ))β1 π0 (1 β π§2 π0) , (42)
maps D into D \ {0}. Hence, there exists a holomorphicfunction π€ mapping D into H+ such that π€(1) = 0, βπ€(1) =1 > 0, and
π0 (π§) = exp (βπ€ (π§)) , (43)
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6 Journal of Function Spaces
for all π§ β D. In particular, for π₯ = 1β2|π§0| β (β1, 1), we haveexp (βReπ€ (π₯)) = π0 (π₯) =
π0 (((1 β π₯) /2) π0)1 β 2 log ((1 + π₯) /2)=
π0 (π§0)1 β 2 log (1 β π§0)= πΉ (π§0)1 β 2 log (1 β π§0)= exp(β 2 π§01 β π§0)= exp(β21 β π₯1 + π₯) .
(44)
Thus,
Reπ€ (π₯) = 21 β π₯1 + π₯ , (45)for some π₯ β (β1, 1) and, by Lemma 2, we conclude that
π€ (π§) = 21 β π§1 + π§ (46)for all π§ β D. Therefore,
π0 (1 β π§2 π0) = (1 β 2 log(1 + π§2 ))π0 (π§)= (1 β 2 log(1 + π§2 )) exp(β21 β π§1 + π§) ,
(47)
for all π§ β D. Hence, changing (1βπ§)/2 by π§, which belongs toπ·(1/2, 1/2), and using the identity principle for holomorphicfunctions, we obtain that
π0 (π§π0) = πΉ (π§) , (48)for all π§ β D. This last relation implies that βπΉβlog β€ 1 whichis a contradiction to Proposition 7. This complete the proofof item (1).
(2) Arguing as in the proof of part (1), for |π| = 1 fixed, weset the function
π (π’) = (1 β 2 log(1 + π’2 ))β1 π (1 β π’2 π) , (49)
with π’ β D. We have shown that there exists a holomorphicfunction π€ such that π(π’) = exp{βπ€(π’)} for all π’ β D.Furthermore, π€ satisfies the hypothesis of Juliaβs Lemma(Lemma 1); that is, π€ is a holomorphic function in D βͺ {1},whichmapsD intoH+,π€(1) = 0, and βπ€(1) = π = 1. Hence,for π = (1 β π₯)/(1 + π₯) with π₯ β [0, 1) fixed, π€ maps thehorodisk Ξ(1, π) into the open Euclidean disk with center at(1 β π₯)/(1 + π₯) and radius (1 β π₯)/(1 + π₯). In particular, sinceπ₯ β Ξ(1, π), we have thatπ€(π₯) β π·π₯ = π·((1 β π₯)/(1 + π₯), (1 βπ₯)/(1 + π₯)). Thus, Lemma 4 allows us to write
Re (π (π₯)) = Re (exp (βπ€ (π₯)))β₯ min {Re (exp (βπ§)) : π§ β π·π₯}= exp(β21 β π₯1 + π₯) ,
(50)
for all π₯ β [0, 1). This last inequality is equivalent to writingRe((1 β 2 log(1 + π₯2 ))
β1 π (1 β π₯2 π))β₯ exp (β21 β π₯1 + π₯) ,
(51)
for all π₯ β [0, 1) and from here we haveRe(π (1 β π₯2 π))
β₯ exp(β21 β π₯1 + π₯)(1 β 2 log(1 + π₯2 )) .(52)
Making the change π = (1 β π₯)/2, we obtain π β (0, 1/2] sinceπ₯ β [0, 1) and alsoRe (π (ππ)) β₯ exp (β2 π1 β π) (1 β 2 log (1 β π)) . (53)
We conclude, as before, that
Re (π (π§)) β₯ exp(β2 |π§|1 β |π§|) (1 β 2 log (1 β |π§|)) , (54)for all |π§| β€ 1/2. This shows the inequality in the second partof the theorem.
Now, if there exists a function π0 β π½(β)log such thatReπ(π§0) = πΉ(|π§0|) for some π§0 β π·(0, 1/2), then we candefine π0 = π§0/|π§0| and the function
π0 (π§) = (1 β 2 log(1 + π§2 ))β1 π0 (1 β π§2 π0) , (55)
which maps D into D \ {0}. Hence, as before, there exists aholomorphic functionπ€mappingD intoH+ such thatπ€(1) =0, βπ€(1) = 1 > 0, and π0(π§) = exp(βπ€(π§)) for all π§ β D. Inparticular, for π₯ = 1 β 2|π§0| β [0, 1), we haveRe exp (βπ€ (π₯)) = Reπ0 (π₯) = Reπ0 (((1 β π₯) /2) π0)1 β 2 log ((1 + π₯) /2)
= Reπ0 (π§0)1 β 2 log (1 β π§0)= πΉ (π§0)1 β 2 log (1 β π§0)= exp(β 2 π§01 β π§0)= exp (β21 β π₯1 + π₯) ;
(56)
that is,π€(π₯) is the value inπ·π₯ where Re exp(βπ€(π₯)) attain itsminimum value, but by the proof of Lemma 4, we know thatthis happens if Imπ€(π₯) = 0 andReπ€(π₯) = β2((1βπ₯)/(1+π₯)).Now, by Lemma 2, we conclude thatπ€(π§) = 2((1βπ§)/(1+π§))for all π§ β D. As before, this last fact implies that π0(π§π0) =πΉ(π§) for all π§ β D and therefore βπΉβlog β€ 1 which is acontradiction to Proposition 7. This completes the proof ofitem (2).
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Journal of Function Spaces 7
5. Distortion Theorems for Complex FunctionsinBlog Having Branch Points
In this section we establish a distortion theorem for functionsin the closed unit ball of Blog having branch points andsatisfying a normalized Bloch conditions. More precisely, foreach π β N, we denote by π½(π)log the class of all holomorphicfunctions π β Blog such that π(0) = 0, π(0) = 1, βπβlog β€ 1and if π(π) = 0 for some π β D then π(π)(π) = 0 for allπ = 1, 2, . . . , π. Clearly we have
π½(β)log =ββπ=1
π½(π)log. (57)With these notations, we have the following result.
Theorem 10. For π β N fixed, we set ππ = βπ/(π + 2). Thenfor every π β π½(π)log we have the following:
(1) |π(π§)| β₯ πΉπ(|π§|) = ((ππβ|π§|)/(ππβπ2π|π§|))π(1β2 log(1βππ|π§|)), for all |π§| β€ ππ.There is not a functionπ0 β π½(π)logsuch that |π0(π§0)| = πΉπ(|π§0|) for some π§0 β π·(0, ππ) \{0}.
(2) Reπ(π§) β₯ πΉπ(|π§|) = ((ππ β |π§|)/(ππ β π2π|π§|))π(1 β2 log(1 β ππ|π§|)), for all |π§| β€ βπ(π + 2)/(2π + 1).Furthermore, there is not a function π0 β π½(π)log suchthat Reπ0(π§0) = πΉπ(|π§0|) for some π§0 β π·(0,βπ(π + 2)/(2π + 1)) \ {0}.
Proof. (1) Let us fix |π| = 1, π β N and ππ = βπ/(π + 2) < 1.We set the function
π (π’) = (1 β 2 log (1 β π2π) + 2 log (1 β π2ππ’))β1
β π (ππ β πππ’1 β π2ππ’ π) ,(58)
with π’ β D, where log(π€) denotes the principal logarithmicof the complex number π€ β π·(1, 1). Clearly the function πis holomorphic on D βͺ {1} and π(1) = 1 because π(0) = 1.Also, we have
π (π’) = β (1 β 2 log (1 β π2π) + 2 log (1 β π2ππ’))β2
β ( β2π2π1 β π2ππ’)π (ππ β πππ’1 β π2ππ’ π)
+ (1 β 2 log (1 β π2π) + 2 log (1 β π2ππ’))β1
β π (ππ β πππ’1 β π2ππ’ π)[ππ (π2π β 1)(1 β π2ππ’)2 ] π.
(59)
And hence π(1) = 2π2π/(1 β π2π) = π since π(0) = 1 andπ(0) = 0 (by Lemma 6). Furthermore, since π β π½(π)log andπ(π’0) = 0 if and only if π(((ππ β πππ’0)/(1 β π2ππ’0))π) = 0, we
conclude that all the zeros of the function π have multiplicityat less π.
On the other hand, since βπβlog β€ 1 we haveπ (π’) β€ 1 β 2 log (1 β π2π) + 2 log (1 β π2ππ’)β1
β log( π1 β (ππ β πππ’) / (1 β π2ππ’)2)β€ 1Re (1 β 2 log (1 β π2π) + 2 log (1 β π2ππ’))β log( π1 β (ππ β πππ’) / (1 β π2ππ’)2)
= [log( π(1 β π2π) / (1 β π2ππ’)2)]β1
β log( π1 β (ππ β πππ’) / (1 β π2ππ’)2) < 1,
(60)
for all π’ β D, since1 β π2π1 β π2ππ’
2 < 1 β
ππ β πππ’1 β π2ππ’2 , (61)
for all π’ β D. Hence, we have shown that π(D) β D. InvokingLemma 3, we conclude that |π(π₯)| β₯ π₯π for all π₯ β [0, 1).Therefore, for each π₯ β [0, 1), the following estimation holds:
1 β 2 log (1 β π2π) + 2 log (1 β π2ππ₯)β1
β π (ππ β πππ₯1 β π2ππ₯ π)
β₯ π₯π. (62)
That is,
π (ππ β πππ₯1 β π2ππ₯ π)
β₯ π₯π (1 β 2 log (1 β π2π) + 2 log (1 β π2ππ₯)) ,
(63)
for all π₯ β [0, 1) since ππ β (0, 1).Next, we make the change π = (ππ β πππ₯)/(1 β π2ππ₯). Thenπ β (0, ππ] since π₯ β [0, 1), π₯ = (1/ππ)((ππ β π)/(1 β πππ)) and
we can write
π (ππ) β₯ ( 1ππππ β π1 β πππ)
π
β (1 β 2 log (1 β π2π) + 2 log(1 β π2π 1ππππ β π1 β πππ))
= ( 1ππππ β π1 β πππ)
π (1 β 2 log (1 β πππ)) .(64)
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8 Journal of Function Spaces
Finally, if we set π = π§/|π§| and π = |π§|, we conclude thatπ (π§) β₯ ( 1ππ
ππ β |π§|1 β ππ |π§|)π (1 β 2 log (1 β ππ |π§|))
= (1 β |π§| /ππ1 β ππ |π§| )π (1 β 2 log (1 β ππ |π§|))
= πΉπ (|π§|) ,(65)
for all |π§| β (0, ππ]. This shows the inequality in part (1).The proof of the second part is similar to part (1) of
Theorem 9. If there exists a function π0 β π½(π)log such that|π0(π§0)| = πΉπ(|π§0|) for some π§0 β π·(0, ππ) \ {0}, then we setπ = π§0/|π§0| and the functionπ0 (π’) = (1 β 2 log (1 β π2π) + 2 log (1 β π2ππ’))β1
β π0 (ππ β πππ’1 β π2ππ’ π)(66)
withπ’ β D.Wehave showed thatπ0 satisfies all the hypothesisof Lemma 3. Furthermore, choosing π₯ β [0, 1) such that
ππ β πππ₯1 β π2ππ₯ =π§0 (67)
we obtainπ0 (ππ β π§0ππ β π2π π§0)
=π0 (π₯)
= (1 β 2 log (1 β π2π) + 2 log( 1 β π2π1 β ππ π§0))
β1
β π0 (π§0) = πΉπ (π§0)1 β 2 log (1 β ππ π§0)
= ( ππ β π§0ππ β π2π π§0)π .
(68)
By Lemma 3, we conclude that π0(π§) = π§π for all π§ β D.Hence,
π0 (ππ β πππ§1 β π2ππ§ π)= (1 β 2 log (1 β π2π) + 2 log (1 β π2ππ§)) π§π,
(69)
for all π§ β D. Changing (ππ β πππ§)/(1 β π2ππ§) by π§, we obtainthat
π0 (π§π) = (1 β 2 log (1 β πππ§)) ( ππ β π§ππ β π2ππ§)π
= πΉπ (π§) ,(70)
for all π§ β π·(0, ππ) and consequently for all π§ β D. Thislast equality implies that βπΉπβlog = βπ0βlog β€ 1 which is acontradiction to Proposition 8.
(2) As before, for π β N, we set ππ = βπ/(π + 2), we fix|π| = 1, and we define the functionπ (π’) = (1 β 2 log (1 β π2π) + 2 log (1 β π2ππ’))β1
β π (ππ β πππ’1 β π2ππ’ π) ,(71)
with π’ β D. In the first part we have shown that this functionsatisfies the hypothesis of Lemma 3. Hence Reπ(π₯) β₯ π₯π forall (π β 1)/(π + 1) β€ π₯ < 1. Therefore
Re((1 β 2 log (1 β π2π) + 2 log (1 β π2ππ₯))β1
β π (ππ β πππ₯1 β π2ππ₯ π)) β₯ π₯π,
(72)
and thus we have
(1 β 2 log (1 β π2π) + 2 log (1 β π2ππ₯))β1
β Re(π (ππ β πππ₯1 β π2ππ₯ π)) β₯ π₯π, (73)
which is the same as
Re(π (ππ β πππ₯1 β π2ππ₯ π))
β₯ π₯π log( π(1 β π2π) / (1 β π2ππ₯)2) .(74)
As before, we make the change π = (ππ β πππ₯)/(1 β π2ππ₯); then0 < π β€ βπ(π + 2)/(2π + 1), π₯ = (1/ππ)((ππ β π)/(1 β πππ)),and
Reπ (ππ) β₯ ( 1ππππ β π1 β πππ)
π
β log( π[(1 β π2π) / (1 β π2π (1/ππ) ((ππ β π) / (1 β πππ)))]2)
= ( 1ππππ β π1 β πππ)
π (1 β 2 log (1 β πππ)) .(75)
Setting π = π§/|π§| and π = |π§|, we conclude thatReπ (π§) β₯ ( 1ππ
ππ β |π§|1 β ππ |π§|)π (1 β 2 log (1 β ππ |π§|)) , (76)
for all |π§| β€ βπ(π + 2)/(2π + 1). This shows the inequality inpart (2).
If there exists a function π0 β π½(π)log such that Reπ0(π§0) =πΉπ(|π§0|) for some π§0 β π·(0,βπ(π + 2)/(2π + 1)) \ {0}, then weset π = π§0/|π§0| and the function
π0 (π’) = (1 β 2 log (1 β π2π) + 2 log (1 β π2ππ’))β1
β π0 (ππ β πππ’1 β π2ππ’ π) ,(77)
-
Journal of Function Spaces 9
withπ’ β D.Wehave showed thatπ0 satisfies all the hypothesisof Lemma 3. Furthermore, choosing π₯ β [0, 1) such that
ππ β πππ₯1 β π2ππ₯ =π§0 , (78)
we obtain
π₯ = 1ππππ β π§01 β ππ π§0 , (79)
and since 0 < |π§0| β€ ππ((π + 2)/(2π + 1)) we can see that(π β 1)/(π + 1) β€ π₯ < 1. Furthermore,Reπ0 ( ππ β
π§0ππ β π2π π§0) = Reπ0 (π₯)
= (1 β 2 log (1 β π2π) + 2 log( 1 β π2π1 β ππ π§0))
β1
β Reπ0 (π§0) = πΉπ (π§0)1 β 2 log (1 β ππ π§0)
= ( ππ β π§0ππ β π2π π§0)π .
(80)
By Lemma 3, we conclude that π0(π§) = π§π for all π§ β D.Hence, as before, this last fact implies that βπΉπβlog = βπ0βlog β€1 which is a contradiction to Proposition 8.6. Some Estimations for the Schlicht Radius
In this section we present some consequences of the resultsobtained in Sections 4 and 5. We recall that if π is aholomorphic function on D and π§0 β D, ππ (π§0, π) denotethe radius of the largest schlicht disk on the Riemann surfaceπ(D) centered at π(π§0) (a schlicht disk on π(D) centered atπ(π§0) means that π maps an open subset of D containing π§0conformally onto this disk). With this notation, we have thefollowing results.
Corollary 11. If π β π½(β)log , thenππ (0, π) β₯ β«1
0πΉ (|π§|) π |π§|
= β«10exp(β 2π‘1 β π‘) (1 β 2 log (1 β π‘)) ππ‘.
(81)
Proof. From the definition of ππ (0, π), it follows the fact thatthere exists a simply connected domain πΈ β D containing thezero such that π maps πΈ conformally onto an Euclidean diskwith center at π(0) and radius ππ (0, π). This Euclidean diskmust meet the boundary of π(D) because, in other cases, theboundary of the set πΈ is a Jordan curve in the interior of Dand we can find an open set π β D where π is univalent;hence π(π) contain an Euclidean disk with center at π(0)and radius greater than ππ (0, π), which contradict with thedefinition of ππ (0, π). We conclude then that there is a radial
segment Ξ jointing π(0) to the boundary of π(D). Let πΎ beinverse image of Ξ under π; then πΎ joint the point 0 to theboundary of D. Thus, fromTheorem 9, it follows that
ππ (0, π) = β«Ξ|ππ€| = β«
πΎ
π (π§) |ππ§|β₯ β«10πΉ (πΎ (π‘)) πΎ (π‘) ππ‘
= β«10πΉ (πΎ (π‘))
πΎ (π‘) πΎ (π‘)πΎ (π‘) ππ‘β₯ β«10πΉ (πΎ (π‘)) πΎ (π‘) β πΎ
(π‘)πΎ (π‘) ππ‘ = β«1
0πΉ (π) ππ
= β«10exp (β 2π1 β π) (1 β 2 log (1 β π)) ππ β 0.4104136111,
(82)
where we have used Cauchy-Schwarzβs inequality in thefourth line, πΎ(π‘) β πΎ(π‘) is the scalar product of πΎ(π‘) and πΎ(π‘),and we have made the change π = |πΎ(π‘)| = βπΎ(π‘) β πΎ(π‘), whereπ β 1β as π‘ β 1β. This shows the result.
While for functions in the classπ½(π)log wehave the following.Corollary 12. Suppose that π β N is fixed. If π β π½(π)log, then
ππ (0, π) β₯ β«βπ/(π+2)0
πΉπ (π‘) ππ‘= β«βπ/(π+2)0
(1 β π‘/ππ1 β πππ‘ )π (1 β 2 log (1 β πππ‘)) ππ‘.
(83)
Proof. Indeed, arguing as in the proof of Corollary 11, fromTheorem 10, it follows that
ππ (0, π) = β«Ξ|ππ€| = β«
πΎ
π (π§) |ππ§|
β₯ β«βπ/(π+2)0
πΉπ (π) ππ= β«βπ/(π+2)0
( ππ β πππ β π2ππ)π (1 β 2 log (1 β πππ)) ππ‘,
(84)
where we have used that π = |πΎ(π‘)| β ππ as π‘ β 1β. Thisshows the result.
Conflicts of Interest
The authors declare that they have no conflicts of interestregarding the publication of this paper.
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10 Journal of Function Spaces
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