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【趣味数学题】ζ-函数无穷级数

2021-12-31 03:36 作者:AoiSTZ23  | 我要投稿

郑涛(Tao Steven Zheng)著

【问题】

以下三道题是我创造的涉及ζ-函数(Zeta function)的无穷级数问题。

注:(1)ζ-函数的无穷级数(infinite series)表达式是 %5Czeta%20(x)%20%3D%20%5Csum_%7Bn%3D1%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7Bn%5Ex%7D;(2)ζ-函数又称 “黎曼ζ-函数”(Riemann Zeta function)。

题一: 证明 %5Czeta%20(x)%20%3D%20%5Cfrac%7B2%5Ex%7D%7B2%5Ex%20-%201%7D%20%5Csum_%7Bn%3D0%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7B%7B(2n%2B1)%7D%5E%7Bx%7D%7D

题二:已知 %20%5Czeta%20(2)%20%3D%20%5Cfrac%7B%7B%5Cpi%7D%5E%7B2%7D%7D%7B6%7D, 推算 A%20%3D%20%5Csum_%7Bn%3D0%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7B%7B(2n%2B1)%7D%5E%7B2%7D%7DB%20%3D%20%5Csum_%7Bn%3D0%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7B%7B(2n%2B2)%7D%5E%7B2%7D%7D

题三: 推算 %20%5Csum_%7Bn%3D1%7D%5E%7B%5Cinfty%7D%20%7B%5Cleft(%5Cfrac%7B1%7D%7Bn(n%2B1)%7D%20%5Cright)%7D%5E%7B2%7D

【题解】

题一

%5Czeta%20(x)%20%3D%20%5Csum_%7Bn%3D1%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7Bn%5Ex%7D%20%3D%201%20%2B%20%5Cfrac%7B1%7D%7B2%5Ex%7D%20%2B%20%5Cfrac%7B1%7D%7B3%5Ex%7D%20%2B%20%5Cfrac%7B1%7D%7B4%5Ex%7D%20%2B%20%5Cfrac%7B1%7D%7B5%5Ex%7D%20%2B%20...

%5Czeta%20(x)%20%3D%201%20%2B%20%5Cleft(%5Cfrac%7B1%7D%7B2%5Ex%7D%20%2B%20%5Cfrac%7B1%7D%7B4%5Ex%7D%20%2B%20%5Cfrac%7B1%7D%7B8%5Ex%7D%20%2B%20%5Cfrac%7B1%7D%7B16%5Ex%7D%20%2B%20...%20%5Cright)%20%2B%20%5Cleft(%5Cfrac%7B1%7D%7B3%5Ex%7D%20%2B%20%5Cfrac%7B1%7D%7B6%5Ex%7D%20%2B%20%5Cfrac%7B1%7D%7B12%5Ex%7D%20%2B%20%5Cfrac%7B1%7D%7B24%5Ex%7D%20%2B%20...%20%5Cright)%20%2B%20%5Cleft(%5Cfrac%7B1%7D%7B5%5Ex%7D%20%2B%20%5Cfrac%7B1%7D%7B10%5Ex%7D%20%2B%20%5Cfrac%7B1%7D%7B20%5Ex%7D%20%2B%20%5Cfrac%7B1%7D%7B40%5Ex%7D%20%2B%20...%20%5Cright)%20%2B%20...%20

%5Czeta%20(x)%20%3D%201%20%2B%20%5Cfrac%7B1%7D%7B2%5Ex%20-%201%7D%20%2B%20%5Cfrac%7B2%5Ex%7D%7B2%5Ex%20-%201%7D%20%5Csum_%7Bn%3D0%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7B%7B(2n%2B1)%7D%5E%7Bx%7D%7D

%5Czeta%20(x)%20%3D%20%5Cfrac%7B2%5Ex%7D%7B2%5Ex%20-%201%7D%20%2B%20%5Cfrac%7B2%5Ex%7D%7B2%5Ex%20-%201%7D%20%5Csum_%7Bn%3D1%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7B%7B(2n%2B1)%7D%5E%7Bx%7D%7D%20

%5Czeta%20(x)%20%3D%20%5Cfrac%7B2%5Ex%7D%7B2%5Ex%20-%201%7D%20%5Csum_%7Bn%3D0%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7B%7B(2n%2B1)%7D%5E%7Bx%7D%7D


题二
(1)推算 A%20%3D%20%5Csum_%7Bn%3D0%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7B%7B(2n%2B1)%7D%5E%7B2%7D%7D

从题一的结果得

%5Czeta%20(2)%20%3D%20%5Cfrac%7B2%5E2%7D%7B2%5E2%20-%201%7D%20%5Csum_%7Bn%3D0%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7B%7B(2n%2B1)%7D%5E%7B2%7D%7D%20

%5Czeta%20(2)%20%3D%20%5Cfrac%7B4%7D%7B3%7D%20%5Csum_%7Bn%3D0%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7B%7B(2n%2B1)%7D%5E%7B2%7D%7D%20

已知 %20%5Czeta%20(2)%20%3D%20%5Cfrac%7B%7B%5Cpi%7D%5E%7B2%7D%7D%7B6%7D%20,那么

%5Csum_%7Bn%3D0%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7B%7B(2n%2B1)%7D%5E%7B2%7D%7D%20%3D%20%5Cfrac%7B3%7D%7B4%7D%5Ctimes%20%5Cfrac%7B%7B%5Cpi%7D%5E%7B2%7D%7D%7B6%7D%20

%5Csum_%7Bn%3D0%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7B%7B(2n%2B1)%7D%5E%7B2%7D%7D%20%3D%20%5Cfrac%7B%7B%5Cpi%7D%5E%7B2%7D%7D%7B8%7D


(2)推算 B%20%3D%20%5Csum_%7Bn%3D0%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7B%7B(2n%2B2)%7D%5E%7B2%7D%7D

因为 A%20%2B%20B%20%3D%20%5Cfrac%7B%7B%5Cpi%7D%5E%7B2%7D%7D%7B6%7D%20

%5Csum_%7Bn%3D0%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7B%7B(2n%2B2)%7D%5E%7B2%7D%7D%20%2B%20%5Cfrac%7B%7B%5Cpi%7D%5E%7B2%7D%7D%7B8%7D%20%3D%20%5Cfrac%7B%7B%5Cpi%7D%5E%7B2%7D%7D%7B6%7D%20

因此,

%20%5Csum_%7Bn%3D0%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7B%7B(2n%2B2)%7D%5E%7B2%7D%7D%20%3D%20%5Cfrac%7B%7B%5Cpi%7D%5E%7B2%7D%7D%7B24%7D

从这个解法得知 A%20%3D%203B


题三


%20%5Csum_%7Bn%3D1%7D%5E%7B%5Cinfty%7D%20%7B%5Cleft(%5Cfrac%7B1%7D%7Bn(n%2B1)%7D%20%5Cright)%7D%5E%7B2%7D%20%3D%20%5Csum_%7Bn%3D1%7D%5E%7B%5Cinfty%7D%20%7B%5Cleft(%5Cfrac%7B1%7D%7Bn%7D%20-%5Cfrac%7B1%7D%7Bn%2B1%7D%20%5Cright)%7D%5E%7B2%7D

%20%5Csum_%7Bn%3D1%7D%5E%7B%5Cinfty%7D%20%7B%5Cleft(%5Cfrac%7B1%7D%7Bn%7D%20-%5Cfrac%7B1%7D%7Bn%2B1%7D%20%5Cright)%7D%5E%7B2%7D%20%3D%20%5Csum_%7Bn%3D1%7D%5E%7B%5Cinfty%7D%20%5Cleft(%5Cfrac%7B1%7D%7Bn%5E2%7D%20-%5Cfrac%7B2%7D%7Bn(n%2B1)%7D%2B%5Cfrac%7B1%7D%7B%7B(n%2B1)%7D%5E%7B2%7D%7D%20%5Cright)%20%20

已知:

%5Csum_%7Bn%3D1%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7Bn%5E2%7D%20%3D%20%5Czeta(2)%20%3D%20%5Cfrac%7B%7B%5Cpi%7D%5E%7B2%7D%7D%7B6%7D

%5Csum_%7Bn%3D1%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7B%7B(n%2B1)%7D%5E%7B2%7D%7D%20%3D%20%5Czeta(2)%20-1%20%3D%20%5Cfrac%7B%7B%5Cpi%7D%5E%7B2%7D%7D%7B6%7D%20-%201%20

%5Csum_%7Bn%3D1%7D%5E%7B%5Cinfty%7D%20%5Cfrac%7B1%7D%7Bn(n%2B1)%7D%20%3D%20%5Csum_%7Bn%3D1%7D%5E%7B%5Cinfty%7D%20%5Cleft(%5Cfrac%7B1%7D%7Bn%7D%20-%20%5Cfrac%7B1%7D%7Bn%2B1%7D%5Cright)%20%3D%201


所以,

%5Csum_%7Bn%3D1%7D%5E%7B%5Cinfty%7D%20%7B%5Cleft(%5Cfrac%7B1%7D%7Bn(n%2B1)%7D%20%5Cright)%7D%5E%7B2%7D%20%3D%202%5Ctimes%20%5Czeta(2)%20-%201%20-%202%5Ctimes%201%20

%5Csum_%7Bn%3D1%7D%5E%7B%5Cinfty%7D%20%7B%5Cleft(%5Cfrac%7B1%7D%7Bn(n%2B1)%7D%20%5Cright)%7D%5E%7B2%7D%20%3D%202%5Ctimes%20%5Cfrac%7B%7B%5Cpi%7D%5E%7B2%7D%7D%7B6%7D%20-%203%20

%5Csum_%7Bn%3D1%7D%5E%7B%5Cinfty%7D%20%7B%5Cleft(%5Cfrac%7B1%7D%7Bn(n%2B1)%7D%20%5Cright)%7D%5E%7B2%7D%20%3D%20%5Cfrac%7B%7B%5Cpi%7D%5E%7B2%7D%7D%7B3%7D%20-%203


关键词:ζ-函数、无穷级数、黎曼




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