∫ − ∞ ∞ ( x sinh − 1 ( x ) ) 6 d x = a π 2 − c b π 4 + f d π 6
where a , b , c , d , f are positive integers with g cd ( b , c ) = g cd ( d , f ) = 1 . Submit a + b + c + d + f .
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Note that ∫ 0 1 x m ( ln x ) n = ( m + 1 ) n + 1 ( − 1 ) n n ! m , n ∈ N ∪ { 0 } and that we can find rationals α k , j for integers 0 ≤ j ≤ k such that ( n n + k ) ≡ j = 0 ∑ k α k , j ( 2 n + 1 ) j Then X K = ∫ 0 1 ( 1 − x 2 ) K ( ln x ) 6 d x = ∫ 0 1 n ≥ 0 ∑ ( n n + K − 1 ) x 2 n ( ln x ) 6 d x = n ≥ 0 ∑ ( n n + K − 1 ) ( 2 n + 1 ) 7 7 2 0 = 7 2 0 n ≥ 0 ∑ j = 0 ∑ K − 1 α K − 1 , j ( 2 n + 1 ) 7 − j 1 = 7 2 0 j = 0 ∑ K − 1 α K − 1 , j ( 1 − 2 7 − j 1 ) ζ ( 7 − j ) for any imteger 3 ≤ K ≤ 6 .
Making the substitutions u = x 2 + 1 + x and then v = u − 1 we note that ∫ R ( x sinh − 1 x ) 6 d x = 2 ∫ 0 ∞ ( x ln ( x 2 + 1 + x ) ) 6 d x = 6 4 ∫ 1 ∞ ( u 2 − 1 ) 6 u 4 ( u 2 + 1 ) ( ln u ) 6 d u = 6 4 ∫ 0 1 ( 1 − v 2 ) 6 v 4 ( v 2 + 1 ) ( ln v ) 6 d v = 6 4 ( 2 X 6 − 5 X 5 + 4 X 4 − X 3 ) = 6 4 × 7 2 0 ⎣ ⎢ ⎢ ⎢ ⎢ ⎡ ( 2 α 5 , 0 − 5 α 4 , 0 + 4 α 3 , 0 − α 2 , 0 ) 1 2 8 1 2 7 ζ ( 7 ) + ( 2 α 5 , 1 − 5 α 4 , 1 + 4 α 3 , 1 − α 2 , 1 ) 6 4 6 3 ζ ( 6 ) + ( 2 α 5 , 2 − 5 α 4 , 2 + 4 α 3 , 2 − α 2 , 2 ) 3 2 3 1 ζ ( 5 ) + ( 2 α 5 , 3 − 5 α 4 , 3 + 4 α 3 , 3 ) 1 6 1 5 ζ ( 4 ) + ( 2 α 5 , 4 − 5 α 4 , 4 ) 8 7 ζ ( 3 ) + 2 α 5 , 5 4 3 ζ ( 2 ) ⎦ ⎥ ⎥ ⎥ ⎥ ⎤ After a lot of calculation, this gives us ∫ R ( x sinh − 1 x ) 6 d x = 3 π 2 − 2 5 π 4 + 4 0 9 π 6 (the terms in ζ ( 3 ) , ζ ( 5 ) , ζ ( 7 ) all cancel out). Thus the answer is 3 + 5 + 2 + 9 + 4 0 = 5 9 .