Can you generalize it?

Calculus Level 5

n = 0 ( 1 ) n ( 20 n 2 + 8 n + 1 ) 2 12 n ( 2 n n ) 5 \large \sum_{n=0}^{\infty} (-1)^n \dfrac{(20n^2 + 8n +1)}{2^{12 n}} \dbinom{2n}{n}^5

The above sum can be written as 2 a π b \dfrac{2^a}{\pi^b} for positive integers a a and b b . Evaluate a + b a+b .

Notation: ( 2 n n ) = ( 2 n ) ! ( n ! ) 2 \dbinom{2n}{n} = \dfrac{(2n)!}{(n!)^2} denotes the central binomial coefficient .


The answer is 5.

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1 solution

Carsten Meyer
May 19, 2021

Useful identities:

Generalized geometric series: Let n N 0 n\in\mathbb{N}_0 and q C q\in\mathbb{C} . Then k = 0 ( k + n n ) q k = 1 ( 1 q ) k + 1 , q < 1 ( ) \begin{aligned} \sum_{k=0}^\infty \binom{k+n}{n}q^k&=\frac{1}{(1-q)^{k+1}}, &&& |q|&<1 &&&&&(*) \end{aligned}

In the link about central binomial coefficients, there is a useful identity to rewrite them as an integral: n N 0 : ( 2 n n ) = 4 n π R d x ( 1 + x 2 ) n + 1 ( ) \begin{aligned} n&\in\mathbb{N}_0:&\binom{2n}{n}&=\frac{4^n}{\pi}\int_{\mathbb{R}}\frac{dx}{(1+x^2)^{n+1}}&&&&&(**) \end{aligned}


First steps

Let S S be the value of the series and P ( n ) P(n) the numerator polynomial. Rewrite the central binomial coefficient using ( ) (**) : S = n = 0 ( 1 ) n P ( n ) 2 12 n 4 5 n π 5 R 5 d V Q ( x ) n + 1 = 1 π 5 n = 0 P ( n ) ( 1 ) n 4 n R 5 d V Q ( x ) n + 1 , Q ( x ) = k = 1 5 ( 1 + x k 2 ) S=\sum_{n=0}^\infty (-1)^n \frac{P(n)}{2^{12n}} \cdot\frac{4^{5n}}{\pi^5}\int_{\mathbb{R}^5} \frac{dV}{Q(\vec{x})^{n+1}}=\frac{1}{\pi^5}\sum_{n=0}^\infty P(n)\cdot\frac{(-1)^n}{4^n}\int_{\mathbb{R}^5} \frac{dV}{Q(\vec{x})^{n+1}},\qquad Q(\vec{x})=\prod_{k=1}^5(1+x_k^2) By dominated convergence, we interchange summation and integration to get S = 1 π 5 R 5 n = 0 P ( n ) ( 1 ) n 4 n Q ( x ) n + 1 d V S=\frac{1}{\pi^5}\int_{\mathbb{R}^5}\sum_{n=0}^\infty P(n)\cdot\frac{(-1)^n}{4^nQ(\vec{x})^{n+1}}\:dV Rewrite the numerator in terms of binomial coefficients in preparation for ( ) (*) : 20 n 2 + 8 n + 1 = 20 ( n + 1 ) ( n + 2 ) 52 ( n + 1 ) + 13 = 40 ( n + 2 2 ) 52 ( n + 1 1 ) + 13 ( n + 0 0 ) 20n^2+8 n+1=20(n+1)(n+2) - 52(n+1) + 13 = 40\binom{n+2}{2} - 52\binom{n+1}{1} + 13\binom{n+0}{0} The integrand consists of three generalized geometric series: S = 1 π 5 R 5 40 ( 1 + f ( x ) ) 3 52 ( 1 + f ( x ) ) 2 + 13 1 + f ( x ) d V Q ( x ) , f ( x ) : = 1 4 Q ( x ) S=\frac{1}{\pi^5}\int_{\mathbb{R}^5} \frac{40}{(1 + f(\vec{x}))^3}-\frac{52}{(1 + f(\vec{x}))^2}+\frac{13}{1 + f(\vec{x})}\:\frac{dV}{Q(\vec{x})},\qquad f(\vec{x}):=\frac{1}{4Q(\vec{x})} These integrals seem to have an analytic solution - the first component is easy, but afterwards you get radicals. At that point, I used numeric integration and manually checked low powers of pi. If anyone knows of a nice way to analytically solve these integrals (or another way altogether), please let me know!

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