Given a finite set $\mathcal{M}$ of $n \times n$ matrices over the integers, can you express the zero matrix as a product of matrices in $\mathcal{M}$? This is known as the mortality problem. Michael Paterson showed in 1970 that it is undecidable, even for $n=3$. Later it was shown that mortality remains undecidable for $n=3$ and $|\mathcal{M}| = 7$, and for $n=21$ and $|\mathcal{M}| = 2$. Decidability of mortality for $n=2$ is open. If all matrices in $\mathcal{M}$ are nonnegative, the mortality problem becomes decidable, because then it matters only whether matrix entries are $0$ or not. Specifically, view the given matrices as transition matrices of a nondeterministic finite automaton where all states are initial and accepting, and check whether there is a word that is not accepted. This problem is decidable and PSPACE-complete. One can construct cases where the shortest word that is not accepted by the automaton has exponential length. In this post we focus on the nonnegative

# One Idea

Blog by Oxford computer scientist Stefan Kiefer