Horrible infinite sum of reciprocals

Calculus Level 5

( 1 1 2 + 1 4 1 5 + 1 7 1 8 + 1 10 1 11 ) ( 1 1 3 1 4 + 1 6 + 1 7 1 9 1 10 + 1 12 + ) = A + ln B \small \left(1-\dfrac12+\dfrac14-\dfrac15+\dfrac17-\dfrac18+\dfrac1{10}-\dfrac1{11}\cdots\right)-\left(1-\dfrac13-\dfrac14+\dfrac16+\dfrac17-\dfrac19-\dfrac1{10}+\dfrac1{12}+\cdots\right)=\sqrt{A}+\ln B where A A and B B are rational numbers. Find A + B A+B .


The answer is 1.0.

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1 solution

Mark Hennings
Jan 24, 2020

Note that n = 0 N 1 n + z = n = 1 N + 1 1 n γ ψ N ( z ) \sum_{n=0}^N \frac{1}{n+z} \; = \; \sum_{n=1}^{N+1}\frac{1}{n} - \gamma - \psi_N(z) where ψ N ( z ) ψ ( z ) \psi_N(z) \to \psi(z) as N N \to \infty . Here ψ \psi is the digamma function. This result enables us to evaluate the two infinite sums.

X = n = 0 ( 1 3 n + 1 1 3 n + 2 ) = 1 3 ( ψ ( 1 3 ) ψ ( 2 3 ) ) = 1 3 π cot 1 3 π = π 3 3 \begin{aligned} X & = \; \sum_{n=0}^\infty\left(\frac{1}{3n+1} - \frac{1}{3n+2}\right) \; = \; - \tfrac13\big(\psi(\tfrac13) - \psi(\tfrac23)\big) \; = \; \tfrac13\pi\cot\tfrac13\pi \\ & = \; \frac{\pi}{3\sqrt{3}} \end{aligned} Similarly Y = n = 0 ( 1 6 n + 1 1 6 n + 3 1 6 n + 4 + 1 6 n + 6 ) = 1 6 ( ψ ( 1 6 ) ψ ( 1 2 ) ψ ( 2 3 ) + ψ ( 1 ) ) = π 3 3 \begin{aligned} Y & = \; \sum_{n=0}^\infty\left(\frac{1}{6n+1} - \frac{1}{6n+3} - \frac{1}{6n+4} + \frac{1}{6n+6}\right) \\ & = \; -\tfrac16\left(\psi(\tfrac16) - \psi(\tfrac12) - \psi(\tfrac23) + \psi(1)\right) \\ & = \; \frac{\pi}{3\sqrt{3}} \end{aligned} after substituting in standard values for the digamma function. Thus the value of the stated expression is X Y = 0 = 0 + ln 1 X - Y = 0 = \sqrt{0} + \ln1 , making the answer 0 + 1 = 1 0 + 1 = \boxed{1} .

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