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# Hadamard three-circle theorem

Let $f(z)$ be a complex analytic function on the annulus $r_{1}\leq\left\lvert z\right\rvert\leq r_{3}$. Let $M(r)$ be the maximum of $\left\lvert f(z)\right\rvert$ on the circle $\left\lvert z\right\rvert=r$. Then $\log M(r)$ is a convex function of $\log r$. Moreover, if $f(z)$ is not of the form $cz^{\lambda}$ for some $\lambda$, then $\log M(r)$ is a strictly convex as a function of $\log r$.

The conclusion of the theorem can be restated as

$\log\frac{r_{3}}{r_{1}}\log M(r_{2})\leq\log\frac{r_{3}}{r_{2}}\log M(r_{1})+% \log\frac{r_{2}}{r_{1}}\log M(r_{3})$ |

for any three concentric circles of radii $r_{1}<r_{2}<r_{3}$.

Related:

MaximumPrinciple, LogarithmicallyConvexFunction, HardysTheorem

Type of Math Object:

Theorem

Major Section:

Reference

## Mathematics Subject Classification

30A10*no label found*30C80

*no label found*

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