■ある無限級数(その120)

[1]πが出現する無限級数

 (1−1/2^3)+(1/4^3−1/5^3)+(1/7^3−1/8^3)+・・・=4π^3/81√3

 (1−1/2^2−1/3^2+1/4^2)+(1/6^2−1/7^2−1/8^2+1/9^2)+・・・=4π^2/25√5

 (1−1/5^3)+(1/7^3−1/11^3)+(1/13^3−1/17^3)+・・・=π^3/18√3

+・

 (1−1/3^3+1/5^3−1/7^3)+(1/9^3−1/11^3+1/13^3−1/15^3)+・・・=π^3/32 

 (1−1/3^3−1/5^3+1/7^3)+(1/9^3−1/11^3−1/13^3+1/15^3)+・・・=3π^3/64√2

[2]ゼータ関数が出現する無限級数

 ζ(2)=∫(0,∞)x/(exp(x)−1)dx

 ζ(s)Γ(s)=∫(0,∞)x^s-1/(exp(x)−1)dx

 ζ(2)=∫(0,1)∫(0,1)1/(1−xy)dxdy

 ζ(2)=−∫(0,1)log(1−y)/ydy

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[参]若原龍彦「美しい無限級数」プレアデス出版

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