Skew and Infinitary Formal Power Series

We investigate finite-state systems with costs. Departing from classical theory, in this paper the cost of an action does not only depend on the state of the system, but also on the time when it is executed. We first characterize the terminating behaviors of such systems in terms of rational formal power series. This generalizes a classical result of Schützenberger. Using the previous results, we also deal with nonterminating behaviors and their costs. This includes an extension of the Büchi-acceptance condition from finite automata to weighted automata and provides a characterization of these nonterminating behaviors in terms of !-rational formal power series. This generalizes a classical theorem of Büchi.