Sum of Every Power's Reciprocal

Level 2

What is the value of this sum? i = 2 j = 2 1 i j \sum_{i=2}^{\infty}\sum_{j=2}^{\infty}\dfrac{1}{i^j}


The answer is 1.

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1 solution

Bogdan Simeonov
Jan 15, 2014

The sum is equal to n = 2 1 n 2 + 1 n 3 + 1 n 4 . . . = n = 2 1 n 2 . n n 1 = n = 2 1 n . ( n 1 ) \displaystyle\sum_{n=2}^{\infty} \frac{1}{n^2} + \frac{1}{n^3}+\frac{1}{n^4}...=\sum_{n=2}^{\infty}\frac{1}{n^2}.\frac{n}{n-1}=\sum_{n=2}^{\infty} \frac{1}{n.(n-1)} . But 1 ( n 1 ) . n = 1 n 1 1 n \frac{1}{(n-1).n}=\frac{1}{n-1} -\frac{1}{n} and lim n 1 n = 0 \displaystyle\lim_{n \rightarrow \infty } \frac{1}{n}=0 .So the sum is equal to 1 2 1 = 1 \frac{1}{2-1}=\boxed1

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