Friday, July 28, 2017

sequences and series - Peculiar Sum regarding the Reciprocal Binomial Coefficients



Whilst playing around on Wolfram Alpha, I typed in the sum

\sum_{x=0}^\infty \frac{1}{\binom{2x}{x}}=\frac{2}{27}(18+\pi\sqrt 3)
I'm not sure how to derive the answer. My first instinct was to expand the binomial coefficient to get
\sum_{x=0}^\infty \frac{x!^2}{(2x)!}
and then to try using a Taylor Series to get the answer. I thought that if I could find a function f(n) with
f(n)=\sum_{x=0}^\infty \frac{x!^2n^x}{(2x)!}
Then my sum would be equal to f(1). How do I find such a function?



EDIT: I continued on this path and realized that I can use this to set up a recurrence relation for f^{(x)}(0):



f^{(0)}(0)=1

f^{(x)}(0)=\frac{x^2}{2x(2x-1)}f^{(x-1)}(0)



However, I'm not sure how this helps me find f(1)...



Am I on the right track? Can somebody help me finish what I started, or point me towards a better method of calculating this sum?



Thanks!


Answer



Hint. One may observe that
\frac{1}{\binom{2n}{n}}=n\int_0^1 t^{n-1}(1-t)^ndt,\qquad n\ge1, giving
\sum_{n=0}^\infty\frac{1}{\binom{2n}{n}}=1+\int_0^1 \sum_{n=1}^\infty nt^{n-1}(1-t)^n\:dt=1+\int_0^1\frac{t-1}{\left(t^2-t+1\right)^2}dt=\frac{2}{27} \left(18+\sqrt{3} \pi \right) the latter integral is classically evaluated by partial fraction decomposition.


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