x = 1 ∏ ∞ 1 6 ( x 6 + 2 x 3 + 1 ) x 2 ( 1 6 x 4 + 2 4 x 2 + 9 )
The following infinite product can be expressed in the form B A tanh 2 ( C B π ) with A a perfect square and B and C prime numbers. Find A × B × C .
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We can evaluate this one using tables of infinite integrals. We note that n = 1 ∏ ∞ 1 6 ( n 6 + 2 n 3 + 1 ) n 2 ( 1 6 n 4 + 2 4 n 2 + 9 ) = n = 1 ∏ ∞ 1 6 ( n 3 + 1 ) 2 n 2 ( 4 n 2 + 3 ) 2 = n = 1 ∏ ∞ ( 1 + n − 3 ) 2 ( 1 + 4 3 n − 2 ) 2 = ( B A ) 2 where A = n = 1 ∏ ∞ ( 1 + 4 3 n − 2 ) B = n = 1 ∏ ∞ ( 1 + n − 3 ) . Lists of infinite products (Mathworld has just about enough) give A = π 3 2 sinh ( 2 1 π 3 ) B = π 1 cosh ( 2 1 π 3 ) , and so the product is ( B A ) 2 = 3 4 tanh 2 ( 2 1 π 3 ) , making the answer 4 × 3 × 2 = 2 4 .