Inspired by Kishlaya!

Calculus Level 5

n = 1 1 P ( 2 n , n ) = e a π b erf ( c ) 2 \displaystyle \sum_{n=1}^{\infty}{\frac{1}{P(2n,n)}} = \frac{{e}^{a}{\pi}^{b} \ \text{erf}(c)}{2}

Given the above summation with rational numbers a , b , c a,b,c such that a + b + c = M G a + b + c = \frac{M}{G} for coprime positive integers, find M + G M+G

Details and Assumptions

P ( 2 n , n ) P(2n,n) is the permutation of 2 n 2n , n n

erf ( c ) \text{erf}(c) is the error function.

M M and G G are co-prime positive integers.

Megh Choksi asked this problem here .


The answer is 9.

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1 solution

Kartik Sharma
Mar 11, 2015

n = 1 n ! ( 2 n ) ! = n = 1 n ! ( ( n 1 ) ! ) ( ( n 1 ) ! ) ( 2 n ) ! \displaystyle \sum_{n=1}^{\infty}{\frac{n!}{(2n)!}} = \sum_{n=1}^{\infty}{\frac{n!((n-1)!)}{((n-1)!)(2n)!}}

Now, n ! ( ( n 1 ) ! ) ( 2 n ) ! = B ( n + 1 , n ) \displaystyle \frac{n!((n-1)!)}{(2n)!} = B(n+1,n)

n = 1 B ( n + 1 , n ) ( n 1 ) ! \displaystyle \sum_{n=1}^{\infty}{\frac{B(n+1,n)}{(n-1)!}}

B ( n + 1 , n ) = 0 1 t n ( 1 t ) n 1 \displaystyle B(n+1,n) = \int_{0}^{1}{{t}^{n}{(1-t)}^{n-1}}

n = 1 0 1 t n ( 1 t ) n 1 ( n 1 ) ! \displaystyle \sum_{n=1}^{\infty}{\frac{\int_{0}^{1}{{t}^{n}{(1-t)}^{n-1}}}{(n-1)!}}

0 1 n = 1 t n ( 1 t ) n 1 ( n 1 ) ! \displaystyle \int_{0}^{1}{\sum_{n=1}^{\infty}{\frac{{t}^{n}{(1-t)}^{n-1}}{(n-1)!}}}

0 1 1 1 t n = 1 t ( 1 t ) n ( n 1 ) ! \displaystyle \int_{0}^{1}{\frac{1}{1-t}\sum_{n=1}^{\infty}{\frac{{t(1-t)}^{n}}{(n-1)!}}}

n = 1 t ( 1 t ) n ( n 1 ) ! = t ( 1 t ) e t ( 1 t ) \displaystyle \sum_{n=1}^{\infty}{\frac{{t(1-t)}^{n}}{(n-1)!}} = t(1-t){e}^{t(1-t)}

0 1 t ( 1 t ) e t ( 1 t ) 1 t \displaystyle \int_{0}^{1}{\frac{t(1-t){e}^{t(1-t)}}{1-t}}

0 1 t e t ( 1 t ) \displaystyle \int_{0}^{1}{t{e}^{t(1-t)}}

0 1 t e t t 2 1 4 + 1 4 \displaystyle \int_{0}^{1}{t{e}^{t-{t}^{2}-\frac{1}{4} + \frac{1}{4}}}

e 1 4 ( 0 1 t e ( t 1 / 2 ) 2 ) \displaystyle {e}^{\frac{1}{4}}(\int_{0}^{1}{t{e}^{{-(t-1/2)}^{2}}})

Substituting t 1 / 2 = x t-1/2 = x ,

e 1 / 4 ( 1 / 2 1 / 2 x e x 2 + 1 2 ( 1 / 2 1 / 2 e x 2 ) ) \displaystyle {e}^{1/4}(\int_{-1/2}^{1/2}{x{e}^{-{x}^{2}}} + \frac{1}{2}(\int_{-1/2}^{1/2}{{e}^{-{x}^{2}}}))

1 / 2 1 / 2 x e x 2 = e 1 / 4 2 + e 1 / 4 2 = 0 \displaystyle \int_{-1/2}^{1/2}{x{e}^{-{x}^{2}}} = -\frac{{e}^{-1/4}}{2} +\frac{{e}^{-1/4}}{2} = 0

1 2 ( 1 / 2 1 / 2 e x 2 ) = 1 2 2 0 1 / 2 e x 2 \displaystyle \frac{1}{2}(\int_{-1/2}^{1/2}{{e}^{-{x}^{2}}}) = \frac{1}{2}2\int_{0}^{1/2}{{e}^{-{x}^{2}}}

= π 1 / 2 2 e r f ( 1 / 2 ) \displaystyle = \frac{{\pi}^{1/2}}{2}*erf(1/2)

And yes, we have e 1 / 4 {e}^{1/4} in front as well, so, our answer becomes

= e 1 / 4 π 1 / 2 e r f ( 1 / 2 ) 2 \displaystyle = \boxed{\frac{{e}^{1/4}{\pi}^{1/2}erf(1/2)}{2}}

Therefore 1 4 + 1 2 + 1 2 = 5 4 \displaystyle \frac{1}{4} + \frac{1}{2} + \frac{1}{2} = \frac{5}{4} , hence 5 + 4 = 9 5 + 4 = \boxed{9}

Nice question and nice solution (I did the same too :P)

Also I am working on "A tribute to an ingenius mind -Kishlaya Jaiswal - Part 2" which uses double summation and gamma function , just thought I should let you know .

A Former Brilliant Member - 6 years, 3 months ago

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I will wait for it! :) And you are back? Aren't you?

Kartik Sharma - 6 years, 3 months ago

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Next I have my Maths test on Wednesday , so I guess I'll be free till then!

A Former Brilliant Member - 6 years, 3 months ago

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@A Former Brilliant Member Oh! I too have maths on Tues!

Kartik Sharma - 6 years, 3 months ago

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@Kartik Sharma Best of Luck !

P.S. Not that you need it !

A Former Brilliant Member - 6 years, 3 months ago

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@A Former Brilliant Member You are right. I don't need luck, that is impossible to achieve. As in 'Atmatrana' by Rabindranath Tagore, he doesn't wish from his God not giving him failure but he wishes Him to give him strength to get through the failure. Because success is just the final step of all the failures. So I wish that you get the strength to get through.

Kartik Sharma - 6 years, 3 months ago

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@Kartik Sharma I never noticed this comment . Lol

Or just do

n = 1 n ! ( 2 n ) ! = n = 1 1 ( 2 n ) ! 0 e t t n d t = 0 e t cosh t d t \sum_{n=1}^{\infty} \frac{n!}{(2n)!} = \sum_{n=1}^{\infty} \frac{1}{(2n)!} \int_{0}^{\infty} e^{-t} t^{n} \ dt = \int_{0}^{\infty} e^{-t} \cosh \sqrt{t} \ dt

Shortens the proof a lot.

Jake Lai - 6 years, 1 month ago

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Yeah! Nice! Thanks!

Kartik Sharma - 6 years ago

this one's good

Aman Rajput - 5 years, 11 months ago

Thanks! @megh choksi @Kishlaya Jaiswal @Siddhartha Srivastva

Kartik Sharma - 6 years, 3 months ago

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That was great! Exactly the way how I did it.

And also a great thanks to you ¨ \ddot \smile

EDIT : just a side note : for using large brackets you can use "\left(" for left bracket, alongwith "\right)" for right bracket. Also use "\ [ ... \ ]" instead of "\ ( ... \ )" when you don't to display inline math and want your equations to stand out.

Kishlaya Jaiswal - 6 years, 3 months ago

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Oh thanks for the advice! I will take care of it. BTW, can there be any other approaches to the problem? I am really glad that you liked that problem. And yeah, you shouldn't have to thank me as it is not original, you should thank Megh for it! :)

Kartik Sharma - 6 years, 3 months ago

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@Kartik Sharma Ok! a great thanks to @megh choksi for this nice problem.

Btw, I am "working" on another clean approach.

And now it is proved that a closed form doesn't exists as I earlier conjectured in my previous note (because error function isn't an elementary function).

Kishlaya Jaiswal - 6 years, 3 months ago

@Kartik Sharma ,try to use \left( and \right) ,your solutionis good , but small brackets >.<

Parth Lohomi - 6 years ago

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