Ngô Quốc Anh

January 6, 2011

The Alexandrov-Bol inequality

Filed under: Giải Tích 6 (MA5205) — Tags: , — Ngô Quốc Anh @ 19:55

In the literature, there is an inequality called the Alexandrov-Bol inequality which is frequently used in partial differential equations. Here we just recall its statement without any proof.

Theorem. Let \Omega be a good domain in \mathbb R^2. Assume p \in C^2(\Omega)\cap C^0(\overline \Omega) be a positive function satisfying the elliptic inequality

\displaystyle -\Delta \log p \leqslant p

in \Omega. Then it holds

\displaystyle l^2(\partial\Omega) \geqslant \frac{1}{2} \big(8\pi-m(\Omega)\big)m(\Omega)


\displaystyle l(\partial\Omega)=\int_{\partial\Omega}\sqrt{p}ds


\displaystyle m(\Omega)=\int_\Omega pdx.

An analytic proof was given by C. Bandle aroud 1975 when she assumed p to be real analytic. The above version was due to Suzuki in an elegant paper published in the Ann. Inst. H. Poincare in 1992 [here]. The proof is mainly depended on the isoperimetric inequality for the flat Riemannian surfaces. We refer the reader to the paper by Suzuki for the proof.

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