φ \varphi -tastical Summation!

Calculus Level 5

S = 2 3 n = 0 ( 1 ) n ( 2 n ) ! ( 2 ( 1 + 5 ) ) n ( 2 n + 1 ) n ! n ! S = \dfrac{2}{3}\sum\limits_{n=0}^{\infty} \dfrac{(-1)^n(2n)!}{(2(1 + \sqrt{5}))^n(2n+1)n!n!}

Given that S = A B ( C + D E ) ln ( F G ( H + I J ) ) S = \sqrt{\dfrac{A}{B}(C + D\sqrt{E})}\ln\left(\dfrac{F}{G}(H + I\sqrt{J})\right) , where

  • A , B , C , D , E , F , G , H , I A,B,C,D,E,F,G,H,I are positive integers;
  • gcd ( A , B ) = gcd ( C , D ) = gcd ( F , G ) = gcd ( H , I ) = 1 \gcd(A,B) = \gcd(C,D) = \gcd (F,G) = \gcd (H,I) = 1 ;
  • A , B , E , J A,B,E,J are square-free;

Input the least possible value of A + B + C + D + E + F + G + H + I + J A+B+C+D+E+F+G+H+I+J as your answer.


The answer is 20.

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1 solution

Let's generalise the series

It is known from Generating function and Taylor series that

n = 0 ( 1 ) n ( 2 n ) ! 2 2 n ( n ! ) 2 x 2 n = 1 1 + x 2 \displaystyle\sum_{n=0}^∞ \dfrac{(-1)^n (2n)!}{2^{2n}(n!)^2} x^{2n} = \dfrac{1}{\sqrt{1+x^2}}

Taking integral both sides from 0 0 to a a

n = 0 ( 1 ) n ( 2 n ) ! a 2 n + 1 ( 2 ) 2 n ( 2 n + 1 ) . ( n ! ) 2 = 0 a 1 1 + x 2 d x \sum_{n=0}^∞ \dfrac{(-1)^n (2n)! a^{2n+1}}{(2)^{2n}(2n+1).(n!)^2} =\int_0^a \dfrac{1}{\sqrt{1+x^2}} dx n = 0 ( 1 ) n ( 2 n ) ! a 2 n + 1 2 2 n ( 2 n + 1 ) . ( n ! ) 2 = [ log ( x + x 2 + 1 ) ] 0 a \implies \sum_{n=0}^∞ \dfrac{(-1)^n (2n)! a^{2n+1}}{2^{2n}(2n+1).(n!)^2} =[\log(x+\sqrt{x^2+1})]_0^a = log ( a + a 2 + 1 ) = \log(a+\sqrt{a^2+1} ) Now put a = 2 1 + 5 = 1 ϕ a= \sqrt{\dfrac{2}{1+\sqrt{5}} }=\sqrt{\dfrac{1}{\phi }} The series becomes

2 1 + 5 n = 0 ( 1 ) n ( 2 n ) ! ( 2 ( 1 + 5 ) ) n ( 2 n + 1 ) ( n ! ) 2 = log ( 1 + ϕ ϕ ) = 3 2 log ( 1 + 5 2 ) \displaystyle\sqrt{ \dfrac{2}{1+\sqrt{5}}}\sum_{n=0}^∞ \dfrac{(-1)^n (2n)!}{(2(1+\sqrt{5}))^n (2n+1)(n!)^2} = \log(\frac{1+\phi}{\sqrt{\phi}} )= \dfrac{3}{2} \log(\frac{1+\sqrt{5}}{2} )

So 2 3 n = 0 ( 1 ) n ( 2 n ) ! ( 2 n + 1 ) ( 2 ( 1 + 5 ) ) n ( n ! ) 2 = 1 + 5 2 log ( 1 + 5 2 ) \dfrac{2}{3} \sum_{n=0}^∞ \dfrac{(-1)^n (2n)! }{(2n+1)(2(1+\sqrt{5}))^n (n!)^2 }= \sqrt{\dfrac{1+\sqrt{5}}{2}} \log(\dfrac{1+\sqrt{5}}{2} ) Answer is A + B + C + D + E + F + G + H + I = 20 \boxed{A+B+C+D+E+F+G+H+I= 20}

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