Determinization of Integral Discounted-Sum Automata is Decidable

Abstract

AbstractNondeterministic Discounted-Sum Automata (NDAs) are nondeterministic finite automata equipped with a discounting factor $$\lambda >1$$ λ > 1 , and whose transitions are labelled by weights. The value of a run of an NDA is the discounted sum of the edge weights, where the i-th weight is divided by $$\lambda ^{i}$$ λ i . NDAs are a useful tool for modelling systems where the values of future events are less influential than immediate ones.While several problems are undecidable or open for NDA, their deterministic fragment (DDA) admits more tractable algorithms. Therefore, determinization of NDAs (i.e., deciding if an NDA has a functionally-equivalent DDA) is desirable.Previous works establish that when $$\lambda \in \mathbb {N}$$ λ ∈ N , then every complete NDA, namely an NDA whose states are all accepting and its transition function is complete, is determinizable. This, however, no longer holds when the completeness assumption is dropped.We show that the problem of whether an NDA has an equivalent DDA is decidable when $$\lambda \in \mathbb {N}$$ λ ∈ N (in particular, it is in $$\textsf{EXPSPACE}$$ EXPSPACE and is $$\mathsf {PSPACE-hard}$$ PSPACE - hard ).