,
Joël Ouaknine
,
James Worrell
Creative Commons Attribution 4.0 International license
The matrix semigroup membership problem asks, given square matrices M,M₁,…,M_k of the same dimension, whether M lies in the semigroup generated by M₁,…,M_k. It is classical that this problem is undecidable in general, but decidable in case M₁,…,M_k commute. In this paper we consider the problem of whether, given M₁,…,M_k, the semigroup generated by M₁,…,M_k contains a non-negative matrix. We show that in case M₁,…,M_k commute, this problem is decidable subject to Schanuel’s Conjecture. We show also that the problem is undecidable if the commutativity assumption is dropped. A key lemma in our decidability proof is a procedure to determine, given a matrix M, whether the sequence of matrices (Mⁿ)_{n = 0}^∞ is ultimately nonnegative. This answers a problem posed by S. Akshay [S. Akshay et al., 2022]. The latter result is in stark contrast to the notorious fact that it is not known how to determine, for any specific matrix index (i,j), whether the sequence (Mⁿ)_{i,j} is ultimately nonnegative. Indeed the latter is equivalent to the Ultimate Positivity Problem for linear recurrence sequences, a longstanding open problem.
@InProceedings{dcosta_et_al:LIPIcs.STACS.2024.27,
author = {D'Costa, Julian and Ouaknine, Jo\"{e}l and Worrell, James},
title = {{Nonnegativity Problems for Matrix Semigroups}},
booktitle = {41st International Symposium on Theoretical Aspects of Computer Science (STACS 2024)},
pages = {27:1--27:16},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-311-9},
ISSN = {1868-8969},
year = {2024},
volume = {289},
editor = {Beyersdorff, Olaf and Kant\'{e}, Mamadou Moustapha and Kupferman, Orna and Lokshtanov, Daniel},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2024.27},
URN = {urn:nbn:de:0030-drops-197371},
doi = {10.4230/LIPIcs.STACS.2024.27},
annote = {Keywords: Decidability, Linear Recurrence Sequences, Schanuel’s Conjecture}
}