Binomially Amazing! 3

Calculus Level 5

n = 0 1729 ( 1 ) n ( 1729 n ) 4 n + 2 = 2 A B ! C ! ! \large \sum _{ n=0 }^{ 1729 }{ \frac { { \left( -1 \right) }^{ n }{ 1729 \choose n }}{ 4n+2 } ={ 2 }^{ A }\frac { B! }{ C!! } }

Find A + B + C A+B+C .

Note: The double factorial is defined as

  • For an even positive integer, n ! ! = n × ( n 2 ) × × 4 × 2. n!!=n\times (n-2)\times \cdots\times 4\times 2.

  • For an odd positive integer, n ! ! = n × ( n 2 ) × × 3 × 1. n!!=n\times (n-2)\times \cdots\times 3\times 1.

  • If n = 0 n=0 , 0 ! ! = 1. 0!!=1.


The answer is 6916.

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1 solution

Mark Hennings
Feb 4, 2016

The sum is n = 0 1729 ( 1 ) n 4 n + 2 ( 1729 n ) = n = 0 1729 ( 1 ) n ( 1729 n ) 0 1 x 4 n + 1 d x = 0 1 ( 1 x 4 ) 1729 x d x = 1 2 0 1 ( 1 y 2 ) 1729 d y = 1 2 0 1 2 π cos 3459 θ d θ = 1 2 × 2 1729 1729 ! 3459 ! ! = 2 1728 1729 ! 3459 ! ! \begin{array}{rcl} \displaystyle \sum_{n=0}^{1729} \frac{(-1)^n}{4n+2}{1729 \choose n} & = & \displaystyle \sum_{n=0}^{1729} (-1)^n {1729 \choose n} \int_0^1 x^{4n+1}\,dx \; = \; \displaystyle \int_0^1 (1-x^4)^{1729}x\,dx \\ & = & \displaystyle \tfrac12\int_0^1 (1 - y^2)^{1729}\,dy \; = \; \displaystyle \tfrac12\int_0^{\frac12\pi} \cos^{3459}\theta\,d\theta \\ & = & \displaystyle \frac12 \times \frac{2^{1729} 1729!}{3459!!} \; = \; \frac{2^{1728} 1729!}{3459!!} \end{array} using the substitutions y = x 2 y = x^2 and y = sin θ y = \sin\theta . This makes the answer 1728 + 1729 + 3459 = 6916 1728 + 1729 + 3459 = \boxed{6916} .

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