It is the case that
n = 1 ∑ ∞ 2 n ∗ ( n − 2 1 ) 1 = a ∗ ln ( b + c ) ,
where a , b , c are positive integers with a and c being square-free.
Find a b + c .
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Excellent! This is such an elegant solution, Akshay. Thank you for posting it. :)
Did the same! Nice question! Also try this
The sum is 2 ∗ ln ( 1 + 2 ) , making a b + c = 2 1 + 2 = 8 .
I debated whether to post this as a question or a note, since I'm still working on a proof. It's such a beautiful result, though, that I wanted to get it posted regardless and see if the community has any ideas on an elegant proof.
All I could think of is integrating n = 1 ∑ ∞ x n − 3 / 2 = x ( 1 − x ) 1 by using x → x
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I tried using your approach,I am getting the correct answer but how do we find the constant term of integration,we get the answer only on assuming that the constant term is zero.
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Put x = 0 .
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@Pratik Shastri – First term of The series found becomes ∞ .
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@Akshay Bodhare – Sorry,understood my mistake.
Yes, that works perfectly, and it doesn't require remembering any series representations. Thanks! :)
Can you elaborate on how you got this ?
Thanks for the same .
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n = 1 ∑ ∞ x n = 1 − x x using G.P for ∣ x ∣ < 1
n = 1 ∑ ∞ x n − 2 3 = x ( 1 − x ) 1
Now ,integrating we get,
lo g ( 1 − x 1 + x ) = n = 1 ∑ ∞ n − 2 1 x n − 2 1
And then we put x = 2 1
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@Akshay Bodhare – Thanks Akshay Bodhare ,actually I was wondering about where's the 2 n vanished to , so I got confused
I used the Taylor expansion of arctanh(sqrt(x))/sqrt(x) to get the result and after substituting x=2 we ge the desired result
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Great. That's similar to Akshay's posted solution, except that he started with the series for the log function and essentially derived the series for the arctanh function.
i used integrals on (n-1/2) via 1/A = integral 0 to inf e^(-Ax)dx , A>0
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S = n = 1 ∑ ∞ 2 n ∗ ( n − 2 1 ) 1 = 2 n = 1 ∑ ∞ ( 2 n − 1 ) ( 2 1 ) ( 2 n − 1 )
Now,we know that,
lo g ( 1 + x ) = n = 1 ∑ ∞ ( − 1 ) ( n − 1 ) n x n
Also,
− lo g ( 1 − x ) = n = 1 ∑ ∞ n x n
Adding both gives us,
lo g ( 1 − x 1 + x ) ∗ 2 1 = n = 1 ∑ ∞ 2 n − 1 x ( 2 n − 1 )
Therefore,
S = 2 n = 1 ∑ ∞ ( 2 n − 1 ) ( 2 1 ) ( 2 n − 1 ) = 2 1 2 lo g ( 1 − 2 1 1 + 2 1 )
Rationalizing,the radical,we get,
S = 2 1 2 lo g ( 1 + 2 ) 2 = 2 lo g ( 1 + 2 )