Please consider reopening Example of a PID in which $SL_n \neq E_n$. This is a highly nontrivial and natural question of broad interest which certainly should have an answer somewhere on this site. It is not clear to me what additional context anyone thinks is necessary for the question.

Would you please undelete this post? https://math.stackexchange.com/questions/4906361 The asker deleted it just after receiving my answer.

The Flint Hills series, is the series $$\sum_{n=1}^\infty\frac{1}{n^3\sin^2(n)},$$ and it's an open problem as to whether the series converges. From Corollary 3 and the proof of Corollary 4 of this paper, it seems that it's also unknown if $$\lim_{n\to\infty}\frac{1}{n^3\sin^2(n)}$$ converges. Ge...