Anothe Integral Thing I guess

Calculus Level 5

( sinh 1 ( x ) x ) 6 d x = a π 2 b π 4 c + d π 6 f \large \int_{-\infty }^{\infty } \left(\frac{\sinh ^{-1}(x)}{x}\right)^6 \, dx=a \pi ^2 - \frac{b \pi ^ 4}{c} + \frac{d \pi ^6}{f}

where a , b , c , d , f a, b, c, d, f are positive integers with gcd ( b , c ) = gcd ( d , f ) = 1 \gcd(b,c) = \gcd(d,f) = 1 . Submit a + b + c + d + f a+b+c+d+f .


The answer is 59.

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1 solution

Mark Hennings
Dec 16, 2017

Note that 0 1 x m ( ln x ) n = ( 1 ) n n ! ( m + 1 ) n + 1 m , n N { 0 } \int_0^1 x^m (\ln x)^n \; = \; \frac{(-1)^n n!}{(m+1)^{n+1}} \hspace{2cm} m,n \in \mathbb{N} \cup \{0\} and that we can find rationals α k , j \alpha_{k,j} for integers 0 j k 0 \le j \le k such that ( n + k n ) j = 0 k α k , j ( 2 n + 1 ) j {n+k \choose n} \; \equiv \; \sum_{j=0}^k \alpha_{k,j}(2n+1)^j Then X K = 0 1 ( ln x ) 6 ( 1 x 2 ) K d x = 0 1 n 0 ( n + K 1 n ) x 2 n ( ln x ) 6 d x = n 0 ( n + K 1 n ) 720 ( 2 n + 1 ) 7 = 720 n 0 j = 0 K 1 α K 1 , j 1 ( 2 n + 1 ) 7 j = 720 j = 0 K 1 α K 1 , j ( 1 1 2 7 j ) ζ ( 7 j ) \begin{aligned} X_K & = \; \int_0^1 \frac{(\ln x)^6}{(1-x^2)^K}\,dx \;= \; \int_0^1 \sum_{n \ge 0} {n+K-1 \choose n} x^{2n} (\ln x)^6\,dx \\ & = \; \sum_{n \ge 0} {n+K-1 \choose n} \frac{720}{(2n+1)^7}\; = \; 720\sum_{n \ge 0} \sum_{j=0}^{K-1} \alpha_{K-1,j} \frac{1}{(2n+1)^{7-j}} \\ & = \; 720 \sum_{j=0}^{K-1} \alpha_{K-1,j}\left(1 - \frac{1}{2^{7-j}}\right)\zeta(7-j) \end{aligned} for any imteger 3 K 6 3 \le K \le 6 .

Making the substitutions u = x 2 + 1 + x u = \sqrt{x^2+1} + x and then v = u 1 v = u^{-1} we note that R ( sinh 1 x x ) 6 d x = 2 0 ( ln ( x 2 + 1 + x ) x ) 6 d x = 64 1 u 4 ( u 2 + 1 ) ( u 2 1 ) 6 ( ln u ) 6 d u = 64 0 1 v 4 ( v 2 + 1 ) ( 1 v 2 ) 6 ( ln v ) 6 d v = 64 ( 2 X 6 5 X 5 + 4 X 4 X 3 ) = 64 × 720 [ ( 2 α 5 , 0 5 α 4 , 0 + 4 α 3 , 0 α 2 , 0 ) 127 128 ζ ( 7 ) + ( 2 α 5 , 1 5 α 4 , 1 + 4 α 3 , 1 α 2 , 1 ) 63 64 ζ ( 6 ) + ( 2 α 5 , 2 5 α 4 , 2 + 4 α 3 , 2 α 2 , 2 ) 31 32 ζ ( 5 ) + ( 2 α 5 , 3 5 α 4 , 3 + 4 α 3 , 3 ) 15 16 ζ ( 4 ) + ( 2 α 5 , 4 5 α 4 , 4 ) 7 8 ζ ( 3 ) + 2 α 5 , 5 3 4 ζ ( 2 ) ] \begin{aligned} \int_\mathbb{R} \left(\frac{\sinh^{-1}x}{x}\right)^6\,dx & = \;2 \int_{0}^{\infty} \left(\frac{\ln(\sqrt{x^2+1}+x)}{x}\right)^6\,dx \; = \; 64\int_1^\infty \frac{u^4(u^2+1)}{(u^2-1)^6}(\ln u)^6\,du \\ & = \; 64\int_0^1 \frac{v^4(v^2+1)}{(1-v^2)^6}(\ln v)^6\,dv \; = \; 64\big(2X_6 - 5X_5 + 4X_4 - X_3\big) \\ & = \; 64 \times 720\left[ \begin{array}{l}(2\alpha_{5,0} - 5\alpha_{4,0} + 4\alpha_{3,0} - \alpha_{2,0})\tfrac{127}{128}\zeta(7) \\ + (2\alpha_{5,1} - 5\alpha_{4,1} + 4\alpha_{3,1} - \alpha_{2,1})\tfrac{63}{64}\zeta(6) \\ + (2\alpha_{5,2} - 5\alpha_{4,2} + 4\alpha_{3,2} - \alpha_{2,2})\tfrac{31}{32}\zeta(5)\\ + (2\alpha_{5,3} - 5\alpha_{4,3} + 4\alpha_{3,3})\tfrac{15}{16}\zeta(4) \\ + (2\alpha_{5,4} - 5\alpha_{4,4})\tfrac{7}{8}\zeta(3) + 2\alpha_{5,5}\tfrac34\zeta(2) \end{array}\right] \end{aligned} After a lot of calculation, this gives us R ( sinh 1 x x ) 6 d x = 3 π 2 5 2 π 4 + 9 40 π 6 \int_\mathbb{R} \left(\frac{\sinh^{-1}x}{x}\right)^6\,dx \; = \; 3\pi^2 - \tfrac52\pi^4 + \tfrac{9}{40}\pi^6 (the terms in ζ ( 3 ) , ζ ( 5 ) , ζ ( 7 ) \zeta(3), \zeta(5), \zeta(7) all cancel out). Thus the answer is 3 + 5 + 2 + 9 + 40 = 59 3 + 5 + 2 + 9 + 40 = \boxed{59} .

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