Generalization Of A Problem = Unique Symmetry

Calculus Level 5

Let I ( n ) = 0 1 ln ( 1 x n ) x d x \displaystyle I(n) = \int_0^1 \dfrac{ \ln(1-x^n)}x \, dx . Compute n = 1 I ( n ) n \displaystyle \left \lfloor \; \left | \sum_{n=1}^\infty \dfrac{I(n)}n \right | \; \right\rfloor .


The answer is 2.

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2 solutions

I ( n ) = 0 1 ln ( 1 x n ) x d x By Maclaurin’s series, = 0 1 1 x ( k = 1 x n k k ) d x = 0 1 k = 1 x n k 1 k d x = k = 1 x n k n k 2 0 1 = k = 1 1 n k 2 = ζ ( 2 ) n ζ ( ) is the Riemann zeta function = π 2 6 n \begin{aligned} I(n) & = \int_0^1 \frac {\color{#3D99F6}{\ln(1-x^n)}}x dx & \small \color{#3D99F6}{\text{By Maclaurin's series,}} \\ & = \int_0^1 \frac 1x \left( \color{#3D99F6}{-\sum_{k=1}^\infty \frac {x^{nk}}k} \right) dx \\ & = - \int_0^1 \sum_{k=1}^\infty \frac {x^{nk-1}}k dx \\ & = - \sum_{k=1}^\infty \frac {x^{nk}}{nk^2} \bigg|_0^1 \\ & = - \sum_{k=1}^\infty \frac 1{nk^2} \\ & = - \frac {\color{#3D99F6}{\zeta(2)}}n & \small \color{#3D99F6}{\zeta(\cdot) \text{ is the Riemann zeta function}} \\ & = - \frac {\color{#3D99F6}{\pi^2}}{\color{#3D99F6}{6}n} \end{aligned}

Therefore, n = 1 I ( n ) n = n = 1 π 2 6 n 2 = ( π 2 6 ) 2 = 2 \displaystyle \left \lfloor \left| \sum_{n=1}^\infty \frac {I(n)}n \right| \right \rfloor = \left \lfloor \left| \sum_{n=1}^\infty - \frac {\pi^2}{6n^2} \right| \right \rfloor = \left \lfloor \left|- \left(\frac {\pi^2}6\right)^2 \right| \right \rfloor = \boxed{2}

Abhi Kumbale
May 21, 2016

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