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Lebedev–Milin inequality

They state that if

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

for complex numbers ${\displaystyle \beta _{k}}$ and ${\displaystyle \alpha _{k}}$, and ${\displaystyle n}$ is a positive integer, then

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

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

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

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

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