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Calculus Level 3

n = 0 1 ( 2 n + 1 ) 2 = ? \sum _{ n=0 }^{ \infty }{ \frac { 1 }{ { (2n+1) }^{ 2 } } } = \ ?

Enter your answer to 3 decimal places.


The answer is 1.2337.

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1 solution

U Z
Dec 31, 2014

Learnt from @Pranjal Jain , I am just keeping his solution here,

A concise form is to evaluate the expression in the summation notation itself and using the concepts of the Riemann Zeta function, specifically the value ζ ( 2 ) = π 2 6 \zeta(2)=\dfrac{\pi^2}{6} which is the same as the Basel problem.

S = n = 0 ( 1 ( 2 n + 1 ) 2 ) = n = 1 ( 1 n 2 1 ( 2 n ) 2 ) S = n = 1 ( 1 n 2 1 4 1 n 2 ) S=\sum_{n=0}^\infty \left(\frac{1}{(2n+1)^2}\right)=\sum_{n=1}^\infty \left(\frac{1}{n^2}-\frac{1}{(2n)^2}\right)\\ \implies S=\sum_{n=1}^\infty \left(\frac{1}{n^2}-\frac{1}{4}\cdot \frac{1}{n^2}\right)

Distributing the summation symbol inside the general term of the sequence, we have,

S = ζ ( 2 ) 1 4 ζ ( 2 ) = 3 4 ζ ( 2 ) = 3 π 2 24 = π 2 8 S=\zeta(2)-\frac{1}{4}\zeta(2)=\frac{3}{4}\cdot \zeta(2)=\frac{3\pi^2}{24}=\boxed{\dfrac{\pi^2}{8}}

Prasun Biswas - 6 years, 3 months ago

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