哇嘎嘎,这个定理不错!

Rudin's Real and Complex Analysis, page 118.

$\textbf{6.3 Lemma}$ If $z_1,\cdots,z_N$ are complex numbers then there is a subset $S$ of $\{1,\cdots,N\}$ for which

$\left|\sum\limits_{k\in S}z_k\right|\geq \frac{1}{\pi}\sum\limits_{k=1}^N\left|z_k\right|$.


证明也很巧妙啊!
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