Undecidability of Cost-Bounded Reachability in Priced Probabilistic Timed Automata

Priced Probabilistic Timed Automata (PPTA) extend timed automata with cost-rates in locations and discrete probabilistic branching. The model is a natural combination of Priced Timed Automata and Probabilistic Timed Automata. In this paper we focus on cost-bounded probabilistic reachability for PPTA, which determines if the maximal probability to reach a goal location within a given cost bound (and time bound) exceeds a threshold p ∈ (0, 1]. We prove undecidability of the problem for simple PPTA in 3 variants: with 3 clocks and stopwatch cost-rates or strictly positive cost-rates. Because we encode a 2-counter machine in a new way, we can also show undecidability for cost-rates in Z and only 2 clocks.