Universality and Forall-Exactness of Cost Register Automata with Few Registers

Authors Laure Daviaud , Andrew Ryzhikov

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Laure Daviaud
  • School of Computing Sciences, University of East Anglia, Norwich, UK
Andrew Ryzhikov
  • Department of Computer Science, University of Oxford, UK

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Laure Daviaud and Andrew Ryzhikov. Universality and Forall-Exactness of Cost Register Automata with Few Registers. In 48th International Symposium on Mathematical Foundations of Computer Science (MFCS 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 272, pp. 40:1-40:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)


The universality problem asks whether a given finite state automaton accepts all the input words. For quantitative models of automata, where input words are mapped to real values, this is naturally extended to ask whether all the words are mapped to values above (or below) a given threshold. This is known to be undecidable for commonly studied examples such as weighted automata over the positive rational (plus-times) or the integer tropical (min-plus) semirings, or equivalently cost register automata (CRAs) over these semirings. In this paper, we prove that when restricted to CRAs with only three registers, the universality problem is still undecidable, even with additional restrictions for the CRAs to be copyless linear with resets. In contrast, we show that, assuming the unary encoding of updates, the ∀-exact problem (does the CRA output zero on all the words?) for integer min-plus linear CRAs can be decided in polynomial time if the number of registers is constant. Without the restriction on the number of registers this problem is known to be PSPACE-complete.

Subject Classification

ACM Subject Classification
  • Theory of computation → Quantitative automata
  • cost register automata
  • universality
  • forall-exact problem
  • decidability


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