Saturday, 12 December 2015

sequences and series - Convergence of $sum_{k=1}^infty frac{sin(k(k-1))}{k}$

In connection with the strange behavious of a certain sum in Mathematica (https://mathematica.stackexchange.com/q/210849/16361) I suspected a possible divergence but I could not prove of disprove it.



Here's the question: is the sum




$$s_1=\sum_{k=1}^\infty \frac{\sin\left(k(k-1)\right)}{k}$$



convergent or divergent?



Similarly with



$$s_2=\sum_{k=1}^\infty \frac{\sin(k^2)}{k}$$



Numerical evidence (partial sums) seem to indicate convergence.




EDIT 07.12.19



Actually, the story began one step earlier: I considered this unanswered question Convergence of $\sum_{n=1}^{\infty} \frac{\sin(n!)}{n}$



$$s_3 =\sum_{k=1}^\infty \frac{\sin(k!)}{k}$$



and wanted to simplify it replacing $k!$ with something simpler.

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