On non-decidability of reachability for timed-arc Petri nets

Timed-arc Petri nets are not Turing powerful, because, in particular, they cannot simulate a counter with test on zero. Thus, we could think that this model does not extend significantly the expressiveness of untimed Petri nets. But this is not true; in this paper we show that the differences between them are big enough to make the reachability problem undecidable. We also define dead tokens as those that cannot be used for firing any transitions in the future and we present some particular cases where we can identify them on this kind of timed nets.