bstraction Functor for Named Sets Vincenzo Ciancia 1 Ugo Montanari 1 Department of Computer Science University of Pisa lem of dening fully abstract operational models of name passing calculi has been given some elegant solutions, such as coalgebras over presheaf categories or over nominal sets. These formalisms fail to model garbage collection of unused names, hence they do not have nice properties with respects to nite state algorithms. The category of named sets, on the other hand, was designed for the purpose of supporting ecient algorithms to handle the semantics of name passing calculi. However the theory was developed in a rather ad-hoc fashion (e.g. the existence of a nal coalgebra was only proved in the nite case). In this work duce a name abstraction functor for named sets and show that it provides a simple and eective notion of garbage collection of unused names. Along the way, we survey a number of needed results on the category of permutation algebras, an algebra-theoretic...