On Reachability Problems for Low-Dimensional Matrix Semigroups

We consider the Membership and the Half-Space Reachability problems for matrices in dimensions two and three. Our first main result is that the Membership Problem is decidable for finitely generated sub-semigroups of the Heisenberg group over rational numbers. Furthermore, we prove two decidability results for the Half-Space Reachability Problem. Namely, we show that this problem is decidable for sub-semigroups of $\mathrm{G}\mathrm{L}(2,\mathbb{Z})$ and of the Heisenberg group over rational numbers.