Lebedev–Milin inequality

In mathematics, the Lebedev–Milin inequality is any of several inequalities for the coefficients of the exponential of a power series, found by Lebedev and Milin (1965) and Isaak Moiseevich Milin (1977). It was used in the proof of the Bieberbach conjecture, as it shows that the Milin conjecture implies the Robertson conjecture.

They state that if

\sum _{k\geq 0}\beta _{k}z^{k}=\exp \left(\sum _{k\geq 1}\alpha _{k}z^{k}\right)

for complex numbers β_k and α_k, and n is a positive integer, then

\sum _{k=0}^{\infty }|\beta _{k}|^{2}\leq \exp \left(\sum _{k=1}^{\infty }k|\alpha _{k}|^{2}\right),

\sum _{k=0}^{n}|\beta _{k}|^{2}\leq (n+1)\exp \left({\frac {1}{n+1}}\sum _{m=1}^{n}\sum _{k=1}^{m}(k|\alpha _{k}|^{2}-1/k)\right),

|\beta _{n}|^{2}\leq \exp \left(\sum _{k=1}^{n}(k|\alpha _{k}|^{2}-1/k)\right).

See also exponential formula (on exponentiation of power series).

References

Conway, John B. (1995), Functions of One Complex Variable II, Berlin, New York: Springer-Verlag, ISBN 978-0-387-94460-9, OCLC 32014394
Grinshpan, A. Z. (1999), "The Bieberbach conjecture and Milin's functionals", The American Mathematical Monthly, 106 (3): 203–214, doi:10.2307/2589676, JSTOR 2589676, MR 1682341
Grinshpan, Arcadii Z. (2002), "Logarithmic Geometry, Exponentiation, and Coefficient Bounds in the Theory of Univalent Functions and Nonoverlapping Domains", in Kuhnau, Reiner (ed.), Geometric Function Theory, Handbook of Complex Analysis, Volume 1, Amsterdam: North-Holland, pp. 273–332, ISBN 0-444-82845-1, MR 1966197, Zbl 1083.30017 .
Korevaar, Jacob (1986), "Ludwig Bieberbach's conjecture and its proof by Louis de Branges", The American Mathematical Monthly, 93 (7): 505–514, doi:10.2307/2323021, ISSN 0002-9890, JSTOR 2323021, MR 856290
Lebedev, N. A.; Milin, I. M. (1965), "An inequality", Vestnik Leningrad University. Mathematics, 20 (19): 157–158, ISSN 0146-924X, MR 0186793
Milin, I. M. (1977) [1971], Univalent functions and orthonormal systems, Translations of Mathematical Monographs, 49, Providence, R.I.: American Mathematical Society, pp. iv+202, ISBN 0-8218-1599-7, MR 0369684, Zbl 0342.30006 (Translation of the 1971 Russian edition, edited by P. L. Duren).

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