Find the integrals

Calculus Level 3

0 1 ( ln x 1 x ) 4 d x + 4 0 1 ( ln x 1 x ) 3 d x = a π b ( π d e ) c \int_{0}^{1}{\left(\frac {\ln x}{1-x}\right)}^4dx+4 \int_{0}^{1}{\left(\frac {\ln x}{1-x}\right)}^3dx = \frac {a\pi^b(\pi^d-e)}c

where a a , b b , c c , d d , and e e are positive integers. Find a + b + c + d + e a+b+c+d+e .

Bonus: Find a general formula for 0 1 ( ln x 1 x ) k d x \displaystyle \int_{0}^{1}{\left(\frac {\ln x}{1-x}\right)}^kdx


The answer is 68.

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1 solution

Mark Hennings
May 3, 2018

Note that we have the expansion of the so-called "rising factorial" x ( n ) = j = 0 n 1 ( x + j ) = j = 1 n [ n j ] x j n 1 x^{(n)} \; =\; \prod_{j=0}^{n -1}(x+j) \; = \; \sum_{j=1}^n \left[{n \atop j}\right] x^j \hspace{2cm} n \ge 1 where [ n k ] \left[{n \atop k}\right] is an unsigned Stirling number of the first kind. Also note that ( 1 x ) k 1 = r = 0 ( r + 1 ) ( k ) k ! x r x < 1 , k 1 (1 - x)^{-k-1} \; = \; \sum_{r=0}^\infty \frac{(r+1)^{(k)}}{k!} x^r \hspace{2cm} |x| < 1\,,\, k \ge 1 while 0 1 x u ( ln x ) v d x = ( 1 ) v + 1 v ! ( u + 1 ) v + 1 u , v N { 0 } \int_0^1 x^u (\ln x)^v\,dx \; = \; (-1)^{v+1} \frac{v!}{(u+1)^{v+1}} \hspace{2cm}u,v \in \mathbb{N} \cup \{0\} Thus I k = 0 1 ( ln x 1 x ) k + 1 d x = r = 0 ( r + 1 ) ( k ) k ! 0 1 x r ( ln x ) k + 1 d x = ( 1 ) k + 1 r = 0 ( r + 1 ) ( k ) k ! × ( k + 1 ) ! ( r + 1 ) k + 2 = ( 1 ) k + 1 ( k + 1 ) r = 0 ( r + 1 ) ( k ) ( r + 1 ) k + 2 = ( 1 ) k + 1 ( k + 1 ) r = 0 1 ( r + 1 ) k + 2 j = 1 k [ k j ] ( r + 1 ) j = ( 1 ) k + 1 ( k + 1 ) j = 1 k [ k j ] ζ ( k + 2 j ) \begin{aligned} I_k & = \; \int_0^1 \left(\frac{\ln x}{1-x}\right)^{k+1}\,dx \; = \; \sum_{r=0}^\infty \frac{(r+1)^{(k)}}{k!} \int_0^1 x^r (\ln x)^{k+1}\,dx \\ & = \; (-1)^{k+1}\sum_{r=0}^\infty \frac{(r+1)^{(k)}}{k!} \times \frac{(k+1)!}{(r+1)^{k+2}} \; = \; (-1)^{k+1}(k+1)\sum_{r=0}^\infty \frac{(r+1)^{(k)}}{(r+1)^{k+2}} \\ & = \; (-1)^{k+1}(k+1)\sum_{r=0}^\infty \frac{1}{(r+1)^{k+2}} \sum_{j=1}^k \left[{k \atop j}\right] (r+1)^j \; = \; (-1)^{k+1}(k+1)\sum_{j=1}^k \left[ {k \atop j} \right]\zeta(k+2-j) \end{aligned} Thus I 2 = 3 [ ζ ( 3 ) + ζ ( 2 ) ] = 3 ζ ( 3 ) 1 2 π 2 I 3 = 4 [ ζ ( 4 ) + 3 ζ ( 3 ) + ζ ( 2 ) ] = 4 45 π 4 + 12 ζ ( 3 ) + 2 3 π 2 \begin{aligned} I_2 & = \; -3\left[\zeta(3) + \zeta(2)\right] \; = \; -3\zeta(3) - \tfrac12\pi^2 \\ I_3 & = \; 4\left[\zeta(4) + 3\zeta(3) + \zeta(2)\right] \; = \; \tfrac{4}{45}\pi^4 + 12\zeta(3) + \tfrac23\pi^2 \end{aligned} and so the desired quantity is I 3 + 4 I 2 = 4 45 π 4 4 3 π 2 = 4 π 2 ( π 2 15 ) 45 I_3 + 4I_2 \; = \; \tfrac{4}{45}\pi^4 - \tfrac43\pi^2 \; =\; \frac{4\pi^2(\pi^2 - 15)}{45} making the answer 4 + 2 + 45 + 2 + 15 = 68 4 + 2 + 45 + 2 + 15 = \boxed{68} .

A perfect solution, Thank You

Mrigank Shekhar Pathak - 3 years, 1 month ago

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