Integration, Generalization and Sum

Calculus Level 3

In general for n Z 0 n \in\mathbb Z \geq 0 , I n = 1 ( x 2 + 1 ) n + 1 = ( 2 n ) ! ( 2 n n ! ) 2 π I_n =\int_{-\infty}^{\infty} \dfrac{1}{\,(x^2+1)^{n+1}} = \dfrac{\,(2n)!}{\,(2^n n!)^2}\pi Then n = 0 ( 2 n ) ! ! ( 2 n ) ! I n \sum_{n=0}^{\infty} \frac{\,(2n)!!}{\,(2n)!}I_n can be expressed as π A × e B \pi^A \times e^B , where A A and B B are rational numbers.

Submit your answer as the value of the fraction A B \frac{A}{B} .

Notations:


Inspiration .


The answer is 2.

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1 solution

Michael Mendrin
Aug 24, 2018

The key to solving this is to note that

( 2 n ) ! ! = 2 n n ! (2n)!!=2^n n!

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