Who's up to the challenge? 51

Calculus Level 5

0 1 ( 1 x ) ln ( 1 x ) 1 + x 2 d x = G + a π b c ( ln f ) d g + π ln h j \int _{ 0 }^{ 1 }{ \dfrac { (1-{ x })\ln {(1- x) } }{ 1+{ x }^{ 2 } } \, dx } =-G+\dfrac { { a\pi }^{ b } }{ c }- \dfrac {(\ln f)^d} g +\dfrac { \pi \ln { h } }{ j }

The equation above holds true for positive integers a , b , c , d , f , g , h a,b,c,d,f,g,h and j j , with a , c a,c coprime and both f , h f,h minimized.

Find the value of a + b + c + d + f + g + h + j a+b+c+d+f+g+h+j .

Notation : G G denote Catalan's constant , G = n = 0 ( 1 ) n ( 2 n + 1 ) 2 0.916 \displaystyle G = \sum_{n=0}^\infty \dfrac{ (-1)^n}{(2n+1)^2} \approx 0.916 .


The answer is 125.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Mark Hennings
May 21, 2016

If we define F ( x ) = n 1 H n x n n = 0 x ln ( 1 u ) u ( 1 u ) d u = 1 2 ln 2 ( 1 x ) + L i 2 ( x ) F(x) \; =\; \sum_{n\ge1} \frac{H_n x^n}{n} \; = \; -\int_0^x \frac{\ln(1-u)}{u(1-u)}\,du \; = \; \tfrac12\ln^2(1-x) + \mathrm{Li}_2(x) for x < 1 |x| < 1 , so that (by Abel's Lemma) F ( i ) = lim y 1 F ( y i ) = 1 2 ln 2 ( 1 i ) + L i 2 ( i ) = 1 2 [ 1 2 ln 2 1 4 π i ] 2 + i G 1 48 π 2 = 1 8 ( ln 2 ) 2 5 96 π 2 + i ( G 1 8 π ln 2 ) \begin{array}{rcl} F(i)\; = \; \lim_{y\to1-}F(yi) & = & \displaystyle \tfrac12\ln^2(1-i) + \mathrm{Li}_2(i) \; =\; \tfrac12\big[\tfrac12\ln2 - \tfrac14\pi i\big]^2 + iG - \tfrac{1}{48}\pi^2 \\ & = & \displaystyle \tfrac18(\ln2)^2 - \tfrac{5}{96}\pi^2 + i\big(G - \tfrac18\pi\ln2\big) \end{array} then since 0 1 x 2 n ( 1 x ) ln ( 1 x ) d x = m 1 1 m 0 1 x 2 n + m ( x 1 ) d x = m 1 ( 1 m ( m + 2 n + 2 ) 1 m ( m + 2 n + 1 ) ) = H 2 n + 2 2 n + 2 H 2 n + 1 2 n + 1 \begin{array}{rcl} \displaystyle \int_0^1 x^{2n}(1-x)\ln(1-x)\,dx & = & \displaystyle \sum_{m \ge 1} \frac{1}{m}\int_0^1 x^{2n+m}(x-1)\,dx \\ & = & \displaystyle \sum_{m \ge 1} \left(\frac{1}{m(m+2n+2)} - \frac{1}{m(m+2n+1)}\right) \\ & = & \displaystyle \frac{H_{2n+2}}{2n+2} - \frac{H_{2n+1}}{2n+1} \end{array} for all n 0 n\ge 0 , we see that 0 1 ( 1 x ) ln ( 1 x ) 1 + x 2 d x = n 0 ( 1 ) n ( H 2 n + 2 2 n + 2 H 2 n + 1 2 n + 1 ) = n 0 H 2 n + 2 i 2 n + 2 2 n + 2 + i n 0 H 2 n + 1 i 2 n + 1 2 n + 1 = R e [ F ( i ) ] I m [ F ( i ) ] = G + 5 96 π 2 1 8 ( ln 2 ) 2 + 1 8 π ln 2 \begin{array}{rcl} \displaystyle \int_0^1 \frac{(1-x)\ln(1-x)}{1+x^2}\,dx & = & \displaystyle \sum_{n\ge0}(-1)^n \left( \frac{H_{2n+2}}{2n+2} - \frac{H_{2n+1}}{2n+1} \right) \\ & = & \displaystyle -\sum_{n \ge 0} \frac{H_{2n+2}i^{2n+2}}{2n+2} + i\sum_{n\ge0}\frac{H_{2n+1}i^{2n+1}}{2n+1} \\ & = & \displaystyle -\mathrm{Re}\big[F(i)\big] - \mathrm{Im}\big[F(i)\big] \\ & = & \displaystyle -G + \tfrac{5}{96}\pi^2 - \tfrac18(\ln2)^2 + \tfrac18\pi \ln2 \end{array} making the answer 5 + 2 + 96 + 2 + 2 + 8 + 2 + 8 = 125 5+2+96+ 2 + 2 + 8 + 2 + 8 = \boxed{125} .

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...