Summing Up Reciprocal of Central Binomial Coefficient

Calculus Level 5

n = 1 1 ( 2 n n ) \large \sum_{n=1}^\infty \frac{1}{{2n \choose n}}

Suppose the following infinite sum can be written as a + π b c d , \dfrac{a+\pi b\sqrt{c}}{d},

where a , b , c a,b,c and d d are integers with b b and c c prime. Then compute the value of a + b + c + d a+b+c+d ,


The answer is 41.

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2 solutions

Kishlaya Jaiswal
Jan 31, 2015

We differentiate the special case of Kishlaya's Identity to get d d x ( n = 1 x n n ( 2 n n ) ) = d d x ( 4 x 4 x tan 1 ( x 4 x ) ) \frac{d}{dx}\left(\sum_{n=1}^\infty \frac{x^n}{n{2n \choose n}}\right) = \frac{d}{dx}\left(\sqrt{\frac{4x}{4-x}}\tan^{-1}\left(\sqrt{\frac{x}{4-x}}\right)\right) n = 1 x n 1 ( 2 n n ) = 4 tan 1 x 4 x ( 4 x ) 2 x 4 x + 1 4 x \Rightarrow \sum_{n=1}^\infty \frac{x^{n-1}}{{2n \choose n}} = \frac{4\tan^{-1}\sqrt{\frac{x}{4-x}}}{(4-x)^2\sqrt{\frac{x}{4-x}}} + \frac{1}{4-x}

Setting x = 1 x=1 and simplifying, yields our desired result

n = 1 1 ( 2 n n ) = 9 + 2 3 π 27 \boxed{\sum_{n=1}^\infty \frac{1}{{2n \choose n}} = \frac{9+2\sqrt{3}\pi}{27}}

Tedious differentiation, but great problem!

Jake Lai - 6 years, 4 months ago

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Thanks. And yeah! I agree with you. At my first attempt, I also made a mistake.

But anyways, I found the sum interesting, so I posted it.

Kishlaya Jaiswal - 6 years, 4 months ago

Ur qn took me so long......

Julian Poon - 6 years, 4 months ago

What skill do you want to link this to? If it's already present in the skill map, go to edit problem, and select the corresponding topic-subtopic-chapter-skill.

Calvin Lin Staff - 6 years, 4 months ago
Shashwat Shukla
Jan 31, 2015

My method is nowhere near as general as Kishlaya's as his wonderful identity is a far more powerful/general result. After reading through the wiki for Kishlaya's identity, solving this question using the identity seems like 'nuking a mosquito' to me :D. Which is why, I'd like to outline another approach:

Consider, I ( m , n ) = 0 1 x m ( 1 x ) n d x I(m,n)=\int_{0}^{1}x^m(1-x)^ndx Integrate it by parts once to get: I ( m , n ) = n m + 1 I ( m + 1 , n 1 ) I(m,n)=\frac{n}{m+1}I(m+1,n-1) This step can be repeated for I ( m + 1 , n 1 ) I(m+1,n-1) . Again and again till we reach I ( m + n , 0 ) I(m+n,0) which makes for trivial integration. Finally, what we get is: I ( m , n ) = 0 1 x m ( 1 x ) n d x = 1 ( m + n + 1 ) ( m + n m ) I(m,n)=\int_{0}^{1}x^m(1-x)^ndx=\frac{1}{(m+n+1)\binom{m+n}{m}} . For this question, setting m = n m=n we get: 0 1 ( x ( 1 x ) ) n d x = 1 ( 2 n + 1 ) ( 2 n n ) \int_{0}^{1}(x(1-x))^ndx=\frac{1}{(2n+1)\binom{2n}{n}} . Let, α = x ( 1 x ) \alpha=x(1-x) . Then, 0 1 ( 2 n + 1 ) α n d x = 1 ( 2 n n ) \int_{0}^{1}(2n+1)\alpha^ndx=\frac{1}{\binom{2n}{n}} .

Summing from n=1 to \infty will give us the required expression. Note that the L.H.S is the sum of a GP and an AP-GP in α \alpha .

Completing the proof isn't hard, but it is rather lengthy.

Wow! It's a wonderful observation and solution.

Ofcourse, better than doing that tedious differentiation :)

ok, so now I'll be looking forward to make a problem based on your above derivations

Kishlaya Jaiswal - 6 years, 4 months ago

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Thanks a lot. And I'm trying to derive lots of expressions using your identity. Thanks a lot for posting it as a wiki :)

Shashwat Shukla - 6 years, 4 months ago

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Well, it was all mine pleasure.

And I would love to see new identities and derivations.

Kishlaya Jaiswal - 6 years, 4 months ago

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@Kishlaya Jaiswal Well, I did find a very beautiful result recently. I posted it as a problem. Do try it: The problem that has it all

Shashwat Shukla - 6 years, 4 months ago

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@Shashwat Shukla That's an interesting problem and I am still working hard to find a solution.

I've also got another interesting problem for you to solve - 2015 welcomes Kishlaya's Identity

(Actually, I can't post all of my problems here because some of them get selected for Proofathon Contests. So, keep an eye on Proofathon Contests for other interesting problems.)

Anyways, "The problem that has it all" is really amazing. Keep posting such fantastic problems.

Thanks.

Kishlaya Jaiswal - 6 years, 4 months ago

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@Kishlaya Jaiswal I'm glad that you like it. I tried out your problem too and I liked it a lot. Thanks for posting it. And thanks for the heads-up: I'll follow Proofathon more closely. Keep up the good work. Thanks again :)

Shashwat Shukla - 6 years, 4 months ago

jatin yadav Akshay Bodhare Jake Lai Julian Poon Steven Zheng and everyone else...How did you do it? I'm sure there are many more ways to do this. Please outline your approach. No need for a full solution. Thanks in advance :)

Shashwat Shukla - 6 years, 4 months ago

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Pretty much by the identity (plus differentiation, of course). I was contemplating using the Beta function but I'm a very lazy fellow, so I didn't bother.

Jake Lai - 6 years, 4 months ago

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You sure are one heck of a mathematician for a lazy one :D ...Love your problems...And thanks for replying :)

Shashwat Shukla - 6 years, 4 months ago

Can we do this - n = 1 1 2 n P n \sum_{n=1}^\infty \frac{1}{^{2n} P_{n}}

U Z - 6 years, 4 months ago

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Yes, we can because the series converges. But I doubt if a closed form exists.

Give me a day or two and I'll try to come up with a proof. (coz I've my exams starting from day after tomorrow) :)

Kishlaya Jaiswal - 6 years, 4 months ago

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@Kishlaya Jaiswal @megh choksi This is what Wolfram alpha has to say. But I don't think there's an 'elementary' way to work with it.

Shashwat Shukla - 6 years, 4 months ago

Oh, it's really a coincidence that I did exactly the same!

Kartik Sharma - 6 years, 3 months ago

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