Polygamma at half

Calculus Level 5

If 0 1 ln 1729 ( x ) ( x 1 ) x d x = ( 2 a 1 ) 0 x b e x 1 d x , \int_0^1 \dfrac{\ln^{1729} (x)}{(x-1)\sqrt{x}} \, dx=(2^a-1)\int_0^\infty \dfrac{x^b}{e^x-1} \, dx, find the sum of integers a + b a+b .


The answer is 3459.

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2 solutions

For n Z + n\in \mathbb{Z^+} ,

0 1 ln n x ( x 1 ) x d x = 2 n + 1 0 1 ln n x ( x 2 1 ) d x = 2 n [ 0 1 ln n x 1 x d x + 0 1 ln n x 1 + x d x ] = 2 n [ m = 0 0 1 x m ln n x d x + m = 0 ( 1 ) m 0 1 x m ln n x d x ] = 2 n [ m = 0 n ! ( 1 ) n ( m + 1 ) n + 1 + m = 0 n ! ( 1 ) m + n ( m + 1 ) n + 1 ] = 2 n ( 1 ) n + 1 Γ ( n + 1 ) ζ ( n + 1 ) ( 2 2 n ) = ( 1 ) n + 1 ( 2 n + 1 1 ) Γ ( n + 1 ) ζ ( n + 1 ) = ( 1 ) n + 1 ( 2 n + 1 1 ) 0 x n e x 1 d x \displaystyle \begin{aligned} \int_0^1 \dfrac{\ln^{n}x}{(x-1)\sqrt{x}}\; dx &= 2^{n+1} \int_0^1 \dfrac{\ln^n x}{(x^2-1)}\; dx \\ &= -2^n \left[ \int_0^1 \dfrac{\ln^n x}{1-x}\; dx+\int_0^1 \dfrac{\ln^ n x}{1+x}\; dx \right] \\ &= -2^n \left[ \sum_{m=0}^\infty \int_0^1 x^m \ln^n x\; dx +\sum_{m=0}^\infty (-1)^m \int_0^1 x^m \ln^n x \; dx\right] \\ &= -2^n \left[ \sum_{m=0}^\infty \dfrac{n! (-1)^n}{(m+1)^{n+1}} +\sum_{m=0}^\infty \dfrac{n! (-1)^{m+n}}{(m+1)^{n+1}}\right] \\ &= 2^n (-1)^{n+1}\Gamma(n+1)\zeta(n+1)(2-2^{-n}) \\ &= (-1)^{n+1}(2^{n+1}-1)\Gamma(n+1)\zeta(n+1) \\ &= (-1)^{n+1}(2^{n+1}-1)\int_0^\infty \dfrac{x^n}{e^x-1}\; dx \end{aligned}

This makes the answer 1729 + 1730 = 3459 \boxed{1729+1730=3459} after substituting for n = 1729 n=1729

Aareyan Manzoor
Jun 15, 2017

consider the integral representations of ψ n ( z ) = 0 1 ln n ( x ) x z 1 x 1 d x , Γ ( z ) ζ ( z ) = 0 x z 1 e x 1 d x \psi_n(z)=\int_0^1 \dfrac{\ln^n (x) x^{z-1}}{x-1}dx, \Gamma(z)\zeta(z)=\int_0^\infty \dfrac{x^{z-1}}{e^x-1}dx the lhs is ψ n ( 1 / 2 ) \psi_{n} (1/2) .at n=1729 we use legendre duplication and differentiate it continously to get the identity 2 n + 1 ψ n ( 2 z ) = ψ n ( z ) + ψ n ( z + 1 / 2 ) ψ n ( 1 / 2 ) = ( 2 n + 1 1 ) ψ n ( 1 ) = ( 1 ) n + 1 ( 2 n + 1 1 ) Γ ( n + 1 ) ζ ( n + 1 ) 2^{n+1} \psi_n (2z)=\psi_n(z)+\psi_n (z+1/2)\to \psi_n (1/2)=(2^{n+1}-1)\psi_n(1)=(-1)^{n+1} (2^{n+1}-1) \Gamma(n+1)\zeta(n+1) we can use the aforementioned integrals to get the result 0 1 ln n ( x ) ( x 1 ) x d x = ( 1 ) n + 1 ( 2 n + 1 1 ) 0 x n e x 1 d x \int_0^1 \dfrac{\ln^n (x) }{(x-1)\sqrt{x}}dx=(-1)^{n+1}(2^{n+1}-1)\int_0^\infty \dfrac{x^{n}}{e^x-1}dx putting n=1729 we get the answer.

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