Possibly the strangest calculus problem?

Calculus Level 5

n = 1 0 1 x n 1 ln 2 ( x ) ln ( 1 x ) n 2 d x = π a b \sum_{n=1}^\infty\int_0^1 \frac{x^{n-1}\ln^2(x)\ln(1-x)}{n^2} \,dx=-\frac{\pi^{\large a}}{b}

Find a + b \,\large a+b\, for the following problem


The answer is 2841.

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1 solution

Aman Rajput
Jan 21, 2016

Rewrite the Integral by expressing log ( 1 x ) = m = 1 x m m \log(1-x) =-\sum_{m=1}^{\infty}\frac{x^m}{m}

Therefore , after rearranging terms we get

= n = 1 m = 1 0 1 x m + n 1 log 2 x m n 2 d x \displaystyle = -\sum_{n=1}^{\infty} \sum_{m=1}^{\infty} \int\limits_{0}^{1} \frac{x^{m+n-1}\log^2 x}{mn^2} dx

= n = 1 m = 1 2 ( m + n ) 3 m n 2 \displaystyle = -\sum_{n=1}^{\infty} \sum_{m=1}^{\infty}\frac{2}{(m+n)^3mn^2}

= 2 × ζ ( 6 ) 6 =-2\times \frac{\zeta(6)}{6}

= π 6 2835 =-\frac{\pi^6}{2835}

Huh! This is it? It is so stupid of me! I took that integral as derivatives of Beta function and solved it.

Aditya Kumar - 5 years, 4 months ago

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Hahaha.. Use short method :D

Aman Rajput - 5 years, 4 months ago

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This didn't strike me at all!

Aditya Kumar - 5 years, 4 months ago

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@Aditya Kumar no problem dude your method is also good...

Aman Rajput - 5 years, 4 months ago

How did u get 6 in the denominator plz explain.

Seong Ro - 5 years, 4 months ago

You skipped a lot of steps from the third last line to the second last line.

Pi Han Goh - 5 years, 4 months ago

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