:1-31.

Weighted automata are a generalization of non-deterministic automata where each transition, in addition to an input letter, has also a quantity expressing the weight (e.g. cost or probability) of its execution. As for non-deterministic automata, their behaviours can be expressed in terms of either (weighted) bisimilarity or (weighted) language equivalence.

Coalgebras provide a categorical framework for the uniform study of state-based systems and their behaviours.

In this work, we show that coalgebras can suitably model weighted automata in two different ways: coalgebras on Set (the category of sets and functions) characterize weighted bisimilarity, while coalgebras on Vect (the category of vector spaces and linear maps) characterize weighted language equivalence.

Relying on the second characterization, we show three different procedures for computing weighted language equivalence. The first one consists in a generalization of the usual partition refinement algorithm for ordinary automata. The second one is the backward version of the first one. The third procedure relies on a syntactic representation of rational weighted languages.