View source for Bieberbach conjecture

A hypothesis enunciated in 1916 by L. Bieberbach [[#References|[1]]]: For all functions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b0161501.png" /> of class <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b0161503.png" />, i.e. functions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b0161504.png" /> which are regular and univalent in the disc <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b0161505.png" /> and which have the expansion

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b0161506.png" /></td> </tr></table>

in this disc, one has the estimate <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b0161507.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b0161508.png" />, and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b0161509.png" /> only for the [[Koebe function]]s

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615010.png" /></td> </tr></table>

where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615011.png" /> is a real number. Bieberbach proved his conjecture for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615012.png" />. The problem of finding an accurate estimate of the coefficients for the class <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615013.png" /> is a special case of the [[Coefficient problem|coefficient problem]].

Owing to its simple formulation and profound significance, Bieberbach's conjecture attracted the attention of numerous mathematicians and stimulated the development of different methods in the geometric theory of functions of a complex variable. At the time of writing (1977) the validity of the Bieberbach conjecture had been established for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615014.png" />. It was first proved for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615015.png" /> in 1923 by K. Loewner who introduced the parametric method (cf. [[Parametric representation method|Parametric representation method]]); other proofs of the estimate <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615016.png" /> subsequently appeared, and were based on variational methods, on parametric methods and on the method of the extremal metric. The Bieberbach conjecture for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615017.png" /> was first proved in 1955 by the simultaneous use of variational and parametric methods. In 1960 the estimate <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615018.png" /> was obtained much more simply, with the aid of Grunsky's univalence condition. This estimate was also obtained by variational methods, and with the aid of geometric reasoning; it was once more obtained by using the Grunsky inequalities in matrix form. The validity of Bieberbach's conjecture for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615019.png" /> was demonstrated in 1968 with the aid of the Grunsky inequalities; it was proved for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615020.png" /> in 1972 by variational methods. Among other results obtained in trying to prove the Bieberbach conjecture, the following are worthy of note.

W.K. Hayman [[#References|[4]]] obtained a number of results on the asymptotic behaviour of the coefficients of functions which are <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615021.png" />-sheeted in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615022.png" />, in particular for the class <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615023.png" />. He proved that the limit

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615024.png" /></td> </tr></table>

exists and that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615025.png" /> with an equality sign only for the Koebe functions.

A number of studies are available on the local Bieberbach conjecture, i.e. the conjecture that the Koebe function gives <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615026.png" /> at least for those functions in the class <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615027.png" /> which are close to it in the relevant topology (cf. [[Univalent function|Univalent function]]). It was found that for any <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615028.png" /> there exists a sufficiently small <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615029.png" /> such that for a function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615030.png" /> which satisfies the condition <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615031.png" /> the estimate <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615032.png" /> is valid, and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615033.png" /> only for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615034.png" />.

An estimate of the coefficients for all <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615035.png" />, accurate as regards the order of dependence on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615036.png" />, was first obtained in 1925 by J.E. Littlewood by reducing the estimate for the coefficients to an estimate for the (average) integral of the modulus. More accurate estimates were obtained in 1951 by I.E. Bazilevich (<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615037.png" />), and in 1965 by I.M. Milin (<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615038.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615039.png" />).

The best estimates to date (1977) were obtained in 1972 [[#References|[7]]]:

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615040.png" /></td> </tr></table>

and in 1976 [[#References|[10]]]:

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615041.png" /></td> </tr></table>

For reviews of studies on this subject see [[#References|[2]]], [[#References|[3]]], [[#References|[9]]].

====References====
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  L. Bieberbach,   "Über die Koeffizienten derjenigen Potenzreihen, welche eine schlichte Abbildung des Einheitskreises vermitteln"  ''Sitzungsber. Preuss. Akad. Wiss. Phys-Math. Kl.''  (1916)  pp. 940–955</TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top">  G.M. Goluzin,   "Geometric theory of functions of a complex variable" , ''Transl. Math. Monogr.'' , '''26''' , Amer. Math. Soc.  (1969)  (Translated from Russian)</TD></TR><TR><TD valign="top">[3]</TD> <TD valign="top">  I.M. Milin,   "Univalent functions and orthonormal systems" , ''Transl. Math. Monogr.'' , '''49''' , Amer. Math. Soc.  (1977)  (Translated from Russian)</TD></TR><TR><TD valign="top">[4]</TD> <TD valign="top">  W.K. Hayman,   "The asymptotic behaviour of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615042.png" />-valent functions"  ''Proc. London Math. Soc. (3)'' , '''5'''  (1955)  pp. 257–284</TD></TR><TR><TD valign="top">[5a]</TD> <TD valign="top">  M. Ozawa,   "On the Bieberbach conjecture for the sixth coefficient"  ''Kodai Math. Sem. Rep.'' , '''21'''  (1969)  pp. 97–128</TD></TR><TR><TD valign="top">[5b]</TD> <TD valign="top">  M. Ozawa,   "An elementary proof of the Bieberbach conjecture for the sixth coefficient"  ''Kodai Math. Sem. Rep.'' , '''21'''  (1969)  pp. 129–132</TD></TR><TR><TD valign="top">[6]</TD> <TD valign="top">  R.N. Pederson,   M.M. Schiffer,   "A proof of the Bieberbach conjecture for the fifth coefficient"  ''Arch. Rat. Mech. and Anal.'' , '''45'''  (1972)  pp. 161–193</TD></TR><TR><TD valign="top">[7]</TD> <TD valign="top">  C.H. FitzGerald,   "Quadratic inequalities and coefficient estimates for the fifth coefficient"  ''Arch. Rat. Mech. and Anal.'' , '''46'''  (1972)  pp. 356–368</TD></TR><TR><TD valign="top">[8]</TD> <TD valign="top">  N.A. Shirokov,   "On a regularity theorem of Hayman"  ''J. Soviet Math.'' , '''2''' :  6  (1974)  pp. 693–710  ''Zap. Nauchn. Sem. Leningr. Otd. Mat. Inst. Steklov.'' , '''24'''  (1972)  pp. 182–200</TD></TR><TR><TD valign="top">[9]</TD> <TD valign="top">  I.E. Bazilevich,  , ''Mathematics in the USSR during 40 years: 1917–1957'' , '''1''' , Moscow  (1959)  pp. 444–472  (In Russian)</TD></TR><TR><TD valign="top">[10]</TD> <TD valign="top">  D. Horowitz,   "A refinement estimate for univalent functions"  ''Proc. Amer. Math. Soc.'' , '''54'''  (1976)  pp. 176–178</TD></TR></table>



====Comments====
Actually Bieberbach proved <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615043.png" /> and then asked in a footnote if perhaps generally <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615044.png" />, cf. [[#References|[1]]].

In the laborious progress on the Bieberbach conjecture from 1950 until 1975, M. Schiffer's variational methods played a major role. For a detailed history, see Duren's book [[#References|[a3]]].

The case <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615045.png" /> was proved by P.R. Garabedian and M.M. Schiffer in 1955.

In 1984 the Bieberbach conjecture was established in complete generality by the French-born U.S. mathematician Louis de Branges [[#References|[a1]]], [[#References|[a2]]]. In fact, he proved a remarkable hypothesis of the Soviet mathematicians N.A. Lebedev and I.M. Milin which was known to be even stronger than the Bieberbach conjecture, cf. [[#References|[3]]], [[#References|[a3]]]. The Lebedev–Milin hypothesis asserted that for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615046.png" />

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615047.png" /></td> <td valign="top" style="width:5%;text-align:right;">(*)</td></tr></table>

Here the numbers <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615048.png" /> are the logarithmic coefficients of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615049.png" />, defined by

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615050.png" /></td> </tr></table>

Every function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615051.png" /> in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615052.png" /> is the starting point <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615053.png" /> of a Loewner parametric family <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615054.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615055.png" />, of univalent functions. The latter map the disc onto a continuously-increasing family of domains with limit <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615056.png" />. Normalized so that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615057.png" /> is in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615058.png" />, the functions of a Loewner family satisfy Loewner's partial differential equation

<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615059.png" /></td> </tr></table>

where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615060.png" /> [[#References|[a4]]], [[#References|[a3]]]. De Branges introduced functionals <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615061.png" /> corresponding to the logarithmic coefficients <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615062.png" /> for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615063.png" />, replacing the weight factors <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615064.png" /> in (*) by undetermined weights <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615065.png" /> with initial value <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615066.png" />. By ingenious use of Loewner's differential equation he could show that the weights <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615067.png" /> may be chosen such that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615068.png" /> for all <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615069.png" /> and all <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615070.png" />. The proof also used a sophisticated positivity result for hypergeometric functions due to R. Askey and G. Gasper. Since <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615071.png" /> as <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615072.png" /> it followed that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615073.png" />. The latter inequality established the Lebedev–Milin hypothesis (*) and thereby also Bieberbach's conjecture <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b016/b016150/b01615074.png" />. The case of equality was dealt with both by de Branges [[#References|[a2]]] and by C.H. FitzGerald and C. Pommerenke [[#References|[a5]]].

====References====
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  L. de Branges,   "A proof of the Bieberbach conjecture"  ''Preprint E-5–84. Steklov Math. Inst. Leningrad''  (1984)  pp. 1–21</TD></TR><TR><TD valign="top">[a2]</TD> <TD valign="top">  L. de Branges,   "A proof of the Bieberbach conjecture"  ''Acta Math.'' , '''154'''  (1985)  pp. 137–152</TD></TR><TR><TD valign="top">[a3]</TD> <TD valign="top">  P.L. Duren,   "Univalent functions" , Springer  (1983)  pp. Sect. 10.11</TD></TR><TR><TD valign="top">[a4]</TD> <TD valign="top">  C. Pommerenke,   "Univalent functions" , Vandenhoeck &amp; Ruprecht  (1975)</TD></TR><TR><TD valign="top">[a5]</TD> <TD valign="top">  C.H. FitzGerald,   C. Pommerenke,   "The de Branges theorem on univalent functions"  ''Trans. Amer. Math. Soc.'' , '''290'''  (1985)  pp. 683–690</TD></TR></table>