They look similar (2)

Calculus Level 4

n = 1 1 n 2 2 n = A π B C D ( ln E ) F G \sum_{n=1}^\infty\frac1{n^22^n}=\frac{A{\pi}^B}C-\frac{D(\ln E)^F}G A A , B B , C C , D D , E E , F F , and G G are positive integers, with A , C A, C and D , G D,G are positive coprime integers. Find A + B + C + D + E + F + G A+B+C+D+E+F+G .

23 21 22 20

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3 solutions

Aaghaz Mahajan
May 22, 2018

Well, what struck me was the Taylor series of
[-ln(1-x)]/x
Hence, on integrating the function at x = 1/2 gives us the answer....!!

Actually, if you see closely, the expression is simply the Dilogarithm of 0.5.........!!!!

X X
Nov 18, 2018

We know 1 + x 2 + x 2 3 + x 3 4 + . . . = ln ( 1 x ) x \displaystyle1+\frac x2+\frac {x^2}3+\frac{x^3}4+...=\dfrac{-\ln(1-x)}x

Integrate it and get x + x 2 4 + x 3 9 + x 4 16 + . . . = ln ( 1 x ) x d x = f ( x ) \displaystyle x+\frac{x^2}4+\frac{x^3}9+\frac{x^4}{16}+...=\displaystyle\int \frac{-\ln(1-x)}x dx=f(x) , and f ( 0 ) = 0 f(0)=0

So, f ( x ) = 0 x ln ( 1 n ) n d n \displaystyle f(x)=\int_0^x \frac{-\ln(1-n)}n dn , and we have to find f ( 0.5 ) f(0.5)

First, we have f ( 1 ) = π 2 6 = f ( 0.5 ) + ( f ( 1 ) f ( 0.5 ) ) f(1)=\dfrac{\pi^2}6=f(0.5)+(f(1)-f(0.5))


The derivative of ln ( x ) ln ( 1 x ) \ln(x)\ln(1-x) is ln ( x ) 1 x ln ( 1 x ) x \dfrac{-\ln(x)}{1-x}-\dfrac{-\ln(1-x)}x

So, 0 0 ( ln ( x ) ln ( 1 x ) ) 0 0.5 = 0 0.5 ln ( x ) 1 x d x 0 0.5 ln ( 1 x ) x d x \displaystyle\left. _{\color{#FFFFFF}0}^{\color{#FFFFFF}0}(\ln(x)\ln(1-x))\right|_0^{0.5}=\int_{0}^{0.5}\frac{-\ln(x)}{1-x}dx-\int_{0}^{0.5}\frac{-\ln(1-x)}xdx

= 0.5 1 ln ( 1 x ) x d x 0 0.5 ln ( 1 x ) x d x = ( ln ( 0.5 ) ) 2 = ( f ( 1 ) f ( 0.5 ) ) f ( 0.5 ) =\displaystyle\int_{0.5}^{1}\frac{-\ln(1-x)}{x}dx-\int_{0}^{0.5}\frac{-\ln(1-x)}xdx=(\ln(0.5))^2=(f(1)-f(0.5))-f(0.5)


Finally, π 2 6 = f ( 0.5 ) + ( f ( 1 ) f ( 0.5 ) ) \dfrac{\pi^2}6=f(0.5)+(f(1)-f(0.5)) , ( ln ( 0.5 ) ) 2 = ( f ( 1 ) f ( 0.5 ) ) f ( 0.5 ) (\ln(0.5))^2=(f(1)-f(0.5))-f(0.5)

We have π 2 6 ( ln ( 0.5 ) ) 2 = 2 f ( 0.5 ) \dfrac{\pi^2}6-(\ln(0.5))^2=2f(0.5) , f ( 0.5 ) = π 2 12 ( ln ( 2 ) ) 2 2 f(0.5)=\dfrac{\pi^2}{12}-\dfrac{(\ln(2))^2}2

Chew-Seong Cheong
Aug 25, 2018

Relevant wiki: Polylogarithm

The polylogarithm function is defined as Li s ( x ) = n = 1 x n n s \displaystyle \text{Li}_s (x) = \sum_{n=1}^\infty \dfrac {x^n}{n^s} for all complex s s and x 1 |x| \le 1 . Putting s = 2 s=2 and x = 1 2 x=\dfrac 12 , n = 1 1 n 2 2 n = Li 2 ( 1 2 ) = π 2 12 ( ln 2 ) 2 2 \implies \displaystyle \sum_{n=1}^\infty \frac 1{n^2 2^n} = \text{Li}_2 \left(\frac 12 \right) = \dfrac {\pi^2}{12} - \dfrac {(\ln 2)^2}2 . Therefore, A + B + C + D + E + F + G = 1 + 2 + 12 + 1 + 2 + 2 + 2 = 22 A+B+C+D+E+F+G = 1+2+12+1+2+2+2 = \boxed{22} .

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