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So I've been trying to prove that $$\sum^{\infty}_{n=1}\frac{(-1)^{n}\sin(n)}{n}=-\frac{1}{2}$$

I've tried putting various bounds on it to see if I can "squeeze" out the result. Say something like (one of many tried examples): $$ -\frac{1}{n}-\frac{1}{2}\leq \sum^{n}_{k=1}\frac{(-1)^{k}\sin(k)}{k}\leq \frac{1}{n}-\frac{1}{2}$$

I've tried too see if I could find some periodic continuous function in order to use Parseval's theorem, but I can't come up with any that work.

May I please get a hint or some piece of the puzzle for this problem?

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3 Answers 3

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Guide:

Verify by computing the fourier series of $f(x) = \frac{x}{\pi}, -\pi < x<\pi$

$$\frac{x}{\pi} = \frac{2}{\pi}\sum_{n=1}^\infty \frac{(-1)^{n+1}}{n}\sin(nx)$$

and choose an appropriate value of $x$.

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    $\begingroup$ Is there a process for choosing that function? Is it something you just get intuition for? Or is there an obvious reason to choose that function? $\endgroup$ Oct 9, 2018 at 14:16
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    $\begingroup$ hmmm... unfortunately for my case, it's something that I already know. $\endgroup$ Oct 10, 2018 at 3:31
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A direct approach: $$\sum_{n=1}^\infty\frac{(-1)^n\sin n}n=\sum_{n=1}^\infty\frac{(-1)^n\Im (e^{in})}n=-\Im\sum_{n=1}^\infty\frac{(-1)^{n+1}(e^i)^n}n=-\Im\ln(1+e^i)$$ where the Maclaurin series of $\ln(1+z)$ was used. As can be seen by drawing a diagram, $\arg(1+e^i)=\frac12$. This is thus the imaginary part of the logarithm, and thus the original sum is $-\frac12$.

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  • $\begingroup$ The series converges for $ |z| < 1$. Why does it converge for $z=e^i$, which has absolute value $1$? $\endgroup$
    – lhf
    Oct 9, 2018 at 14:23
  • $\begingroup$ @lhf you can show that on the boundary the series converges except at $z=-1$. $\endgroup$ Oct 9, 2018 at 14:24
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    $\begingroup$ @ParclyTaxel One can show this using summation by parts, or more generally by appealing to Dirichlet's test. $\endgroup$
    – Mark Viola
    Oct 9, 2018 at 14:30
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Its is also possible to calculate the series above using the residue theorem from complex analysis. It goes as follows:$$\sum_{i=-\infty}^\infty \text{Res}\left(\pi\csc(\pi z)\cdot\frac {\sin(z)}z,n\right)=-\text{Res}\left(\pi\csc(\pi z)\cdot\frac {\sin(z)}z,0\right)$$

The LHS becomes: $$\sum_{i=-\infty}^\infty\lim_{z\to n}\left[\pi (z-n)\csc(\pi z)\cdot\frac {\sin(z)}z\right]=\sum_{i=-\infty}^\infty(-1)^n \frac {\sin(n)}n=2\sum_{i=1}^\infty(-1)^n \frac {\sin(n)}n$$

The RHS becomes: $$\lim_{z\to 0}\left[\pi z\csc(\pi z)\cdot\frac {\sin(z)}z\right]=1$$ Putting all together (LHS $=-$ RHS) you get: $$\sum_{i=1}^\infty(-1)^n \frac {\sin(n)}n=-\frac 12$$

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