n = 1 ∑ ∞ n 2 2 n 1 = C A π B − G D ( ln E ) F A , B , C , D , E , F , and G are positive integers, with A , C and D , G are positive coprime integers. Find A + B + C + D + E + F + G .
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We know 1 + 2 x + 3 x 2 + 4 x 3 + . . . = x − ln ( 1 − x )
Integrate it and get x + 4 x 2 + 9 x 3 + 1 6 x 4 + . . . = ∫ x − ln ( 1 − x ) d x = f ( x ) , and f ( 0 ) = 0
So, f ( x ) = ∫ 0 x n − ln ( 1 − n ) d n , and we have to find f ( 0 . 5 )
First, we have f ( 1 ) = 6 π 2 = f ( 0 . 5 ) + ( f ( 1 ) − f ( 0 . 5 ) )
The derivative of ln ( x ) ln ( 1 − x ) is 1 − x − ln ( x ) − x − ln ( 1 − x )
So, 0 0 ( ln ( x ) ln ( 1 − x ) ) ∣ ∣ 0 0 . 5 = ∫ 0 0 . 5 1 − x − ln ( x ) d x − ∫ 0 0 . 5 x − ln ( 1 − x ) d x
= ∫ 0 . 5 1 x − ln ( 1 − x ) d x − ∫ 0 0 . 5 x − ln ( 1 − x ) d x = ( ln ( 0 . 5 ) ) 2 = ( f ( 1 ) − f ( 0 . 5 ) ) − f ( 0 . 5 )
Finally, 6 π 2 = f ( 0 . 5 ) + ( f ( 1 ) − f ( 0 . 5 ) ) , ( ln ( 0 . 5 ) ) 2 = ( f ( 1 ) − f ( 0 . 5 ) ) − f ( 0 . 5 )
We have 6 π 2 − ( ln ( 0 . 5 ) ) 2 = 2 f ( 0 . 5 ) , f ( 0 . 5 ) = 1 2 π 2 − 2 ( ln ( 2 ) ) 2
Relevant wiki: Polylogarithm
The polylogarithm function is defined as Li s ( x ) = n = 1 ∑ ∞ n s x n for all complex s and ∣ x ∣ ≤ 1 . Putting s = 2 and x = 2 1 , ⟹ n = 1 ∑ ∞ n 2 2 n 1 = Li 2 ( 2 1 ) = 1 2 π 2 − 2 ( ln 2 ) 2 . Therefore, A + B + C + D + E + F + G = 1 + 2 + 1 2 + 1 + 2 + 2 + 2 = 2 2 .
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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.........!!!!