n = 1 ∑ ∞ ( n 2 n ) 1
Suppose the following infinite sum can be written as d a + π b c ,
where a , b , c and d are integers with b and c prime. Then compute the value of a + b + c + d ,
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Tedious differentiation, but great problem!
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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.
Ur qn took me so long......
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.
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 Integrate it by parts once to get: I ( m , n ) = m + 1 n I ( m + 1 , n − 1 ) This step can be repeated for I ( m + 1 , n − 1 ) . Again and again till we reach 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 = ( m + n + 1 ) ( m m + n ) 1 . For this question, setting m = n we get: ∫ 0 1 ( x ( 1 − x ) ) n d x = ( 2 n + 1 ) ( n 2 n ) 1 . Let, α = x ( 1 − x ) . Then, ∫ 0 1 ( 2 n + 1 ) α n d x = ( n 2 n ) 1 .
Summing from n=1 to ∞ will give us the required expression. Note that the L.H.S is the sum of a GP and an AP-GP in α .
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
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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 :)
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Well, it was all mine pleasure.
And I would love to see new identities and derivations.
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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
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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.
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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 :)
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 :)
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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.
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You sure are one heck of a mathematician for a lazy one :D ...Love your problems...And thanks for replying :)
Can we do this - n = 1 ∑ ∞ 2 n P n 1
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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) :)
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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.
Oh, it's really a coincidence that I did exactly the same!
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We differentiate the special case of Kishlaya's Identity to get d x d ( n = 1 ∑ ∞ n ( n 2 n ) x n ) = d x d ( 4 − x 4 x tan − 1 ( 4 − x x ) ) ⇒ n = 1 ∑ ∞ ( n 2 n ) x n − 1 = ( 4 − x ) 2 4 − x x 4 tan − 1 4 − x x + 4 − x 1
Setting x = 1 and simplifying, yields our desired result
n = 1 ∑ ∞ ( n 2 n ) 1 = 2 7 9 + 2 3 π