eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2020-03-04
37:1
37:18
10.4230/LIPIcs.STACS.2020.37
article
Near-Optimal Complexity Bounds for Fragments of the Skolem Problem
Akshay, S.
1
https://orcid.org/0000-0002-2471-5997
Balaji, Nikhil
2
https://orcid.org/0000-0001-9234-0683
Murhekar, Aniket
3
Varma, Rohith
4
Vyas, Nikhil
5
https://orcid.org/0000-0002-4055-7693
IIT Bombay, India
University of Oxford, UK
University of Illinois, Urbana Champaign, Urbana, IL, USA
Indian Institute of Technology Palakkad, India
MIT, Cambridge, MA, USA
Given a linear recurrence sequence (LRS), specified using the initial conditions and the recurrence relation, the Skolem problem asks if zero ever occurs in the infinite sequence generated by the LRS. Despite active research over last few decades, its decidability is known only for a few restricted subclasses, by either restricting the order of the LRS (upto 4) or by restricting the structure of the LRS (e.g., roots of its characteristic polynomial).
In this paper, we identify a subclass of LRS of arbitrary order for which the Skolem problem is easy, namely LRS all of whose characteristic roots are (possibly complex) roots of real algebraic numbers, i.e., roots satisfying x^d = r for r real algebraic. We show that for this subclass, the Skolem problem can be solved in NP^RP. As a byproduct, we implicitly obtain effective bounds on the zero set of the LRS for this subclass. While prior works in this area often exploit deep results from algebraic and transcendental number theory to get such effective results, our techniques are primarily algorithmic and use linear algebra and Galois theory. We also complement our upper bounds with a NP lower bound for the Skolem problem via a new direct reduction from 3-CNF-SAT, matching the best known lower bounds.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol154-stacs2020/LIPIcs.STACS.2020.37/LIPIcs.STACS.2020.37.pdf
Linear Recurrences
Skolem problem
NP-completeness
Weighted automata