Bounds on Irrationality Measures and the Flint-Hills Series

It is unknown whether the Flint-Hills series ${\sum }_{n=1}^{\mathrm{\infty }}\frac{1}{n^3{\sin }^2(n)}$ converges. Alekseyev (2011) connected this question to the irrationality measure of $\pi$, that $\mu (\pi )>\frac{5}{2}$ would imply divergence of the Flint-Hills series. In this paper we established a near-complete converse, that $\mu (\pi )<\frac{5}{2}$ would imply convergence. The associated results on the density of close rational approximations may be of independent interest. The remaining edge case of $\mu (\pi )=\frac{5}{2}$ is briefly addressed, with evidence that it would be hard to resolve.