n = 1 ∑ ∞ P ( 2 n , n ) 1 = 2 e a π b erf ( c )
Given the above summation with rational numbers a , b , c such that a + b + c = G M for coprime positive integers, find M + G
Details and Assumptions
P ( 2 n , n ) is the permutation of 2 n , n
erf ( c ) is the error function.
M and G are co-prime positive integers.
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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 .
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I will wait for it! :) And you are back? Aren't you?
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Next I have my Maths test on Wednesday , so I guess I'll be free till then!
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@A Former Brilliant Member – Oh! I too have maths on Tues!
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@Kartik Sharma – Best of Luck !
P.S. Not that you need it !
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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.
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@Kartik Sharma – I never noticed this comment . Lol
Or just do
n = 1 ∑ ∞ ( 2 n ) ! n ! = n = 1 ∑ ∞ ( 2 n ) ! 1 ∫ 0 ∞ e − t t n d t = ∫ 0 ∞ e − t cosh t d t
Shortens the proof a lot.
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Yeah! Nice! Thanks!
this one's good
Thanks! @megh choksi @Kishlaya Jaiswal @Siddhartha Srivastva
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That was great! Exactly the way how I did it.
And also a great thanks to you ⌣ ¨
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.
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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! :)
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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).
@Kartik Sharma ,try to use \left( and \right) ,your solutionis good , but small brackets >.<
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n = 1 ∑ ∞ ( 2 n ) ! n ! = n = 1 ∑ ∞ ( ( n − 1 ) ! ) ( 2 n ) ! n ! ( ( n − 1 ) ! )
Now, ( 2 n ) ! n ! ( ( n − 1 ) ! ) = B ( n + 1 , n )
n = 1 ∑ ∞ ( n − 1 ) ! B ( n + 1 , n )
B ( n + 1 , n ) = ∫ 0 1 t n ( 1 − t ) n − 1
n = 1 ∑ ∞ ( n − 1 ) ! ∫ 0 1 t n ( 1 − t ) n − 1
∫ 0 1 n = 1 ∑ ∞ ( n − 1 ) ! t n ( 1 − t ) n − 1
∫ 0 1 1 − t 1 n = 1 ∑ ∞ ( n − 1 ) ! t ( 1 − t ) n
n = 1 ∑ ∞ ( n − 1 ) ! t ( 1 − t ) n = t ( 1 − t ) e t ( 1 − t )
∫ 0 1 1 − t t ( 1 − t ) e t ( 1 − t )
∫ 0 1 t e t ( 1 − t )
∫ 0 1 t e t − t 2 − 4 1 + 4 1
e 4 1 ( ∫ 0 1 t e − ( t − 1 / 2 ) 2 )
Substituting t − 1 / 2 = x ,
e 1 / 4 ( ∫ − 1 / 2 1 / 2 x e − x 2 + 2 1 ( ∫ − 1 / 2 1 / 2 e − x 2 ) )
∫ − 1 / 2 1 / 2 x e − x 2 = − 2 e − 1 / 4 + 2 e − 1 / 4 = 0
2 1 ( ∫ − 1 / 2 1 / 2 e − x 2 ) = 2 1 2 ∫ 0 1 / 2 e − x 2
= 2 π 1 / 2 ∗ e r f ( 1 / 2 )
And yes, we have e 1 / 4 in front as well, so, our answer becomes
= 2 e 1 / 4 π 1 / 2 e r f ( 1 / 2 )
Therefore 4 1 + 2 1 + 2 1 = 4 5 , hence 5 + 4 = 9