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Calculus Level 5

x = 1 x 2 ( 16 x 4 + 24 x 2 + 9 ) 16 ( x 6 + 2 x 3 + 1 ) \prod _{ x=1 }^{ \infty }{ \frac { { x }^{ 2 }\left( 16{ x }^{ 4 }+24{ x }^{ 2 }+9 \right) }{ 16\left( { x }^{ 6 }+2{ x }^{ 3 }+1 \right) } }

The following infinite product can be expressed in the form A B tanh 2 ( B C π ) \frac { A }{ B } \tanh ^{ 2 }{ \left( \frac { \sqrt { B } }{ C } \pi \right) } with A { A } a perfect square and B { B } and C { C } prime numbers. Find A × B × C A\times B\times C .


The answer is 24.

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1 solution

Mark Hennings
Jan 20, 2016

We can evaluate this one using tables of infinite integrals. We note that n = 1 n 2 ( 16 n 4 + 24 n 2 + 9 ) 16 ( n 6 + 2 n 3 + 1 ) = n = 1 n 2 ( 4 n 2 + 3 ) 2 16 ( n 3 + 1 ) 2 = n = 1 ( 1 + 3 4 n 2 ) 2 ( 1 + n 3 ) 2 = ( A B ) 2 \prod_{n=1}^\infty \frac{n^2(16n^4 + 24n^2 + 9)}{16(n^6 + 2n^3 + 1)} \; = \; \displaystyle \prod_{n=1}^\infty \frac{n^2(4n^2 + 3)^2}{16(n^3 + 1)^2} \; = \; \prod_{n=1}^\infty \frac{\big(1 + \tfrac34n^{-2}\big)^2}{\big(1 + n^{-3}\big)^2} \; = \; \left(\frac{A}{B}\right)^2 where A = n = 1 ( 1 + 3 4 n 2 ) B = n = 1 ( 1 + n 3 ) . A \; = \; \prod_{n=1}^\infty \big(1 + \tfrac34n^{-2}\big) \qquad \qquad B \; = \; \prod_{n=1}^\infty \big(1 + n^{-3}\big) \;. Lists of infinite products (Mathworld has just about enough) give A = 2 π 3 sinh ( 1 2 π 3 ) B = 1 π cosh ( 1 2 π 3 ) , A \; = \; \tfrac{2}{\pi\sqrt{3}}\sinh\big(\tfrac12\pi\sqrt{3}\big) \qquad \qquad B \; = \; \tfrac{1}{\pi}\cosh\big(\tfrac12\pi\sqrt{3}\big) \;, and so the product is ( A B ) 2 = 4 3 tanh 2 ( 1 2 π 3 ) , \left(\frac{A}{B}\right)^2 \; = \; \tfrac43\tanh^2\big(\tfrac12\pi\sqrt{3}\big) \;, making the answer 4 × 3 × 2 = 24 4 \times 3 \times 2 = \boxed{24} .

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