Abstract. We investigate final coalgebras in nominal sets. This allows us to define types of infinite data with binding for which all constructions automatically respect alpha equivalence. We give applications to the infinitary lambda calculus.
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Abstract. The notion of a meaningless set has been defined for infinitary lambda calculus axiomatically. Standard examples of meaningless sets are sets of terms that have no head normal form, the set of terms without weak head normal form and the set of rootactive terms. The collection of meaningless sets is a lattice. In this paper, we study the way this lattices decompose as union of more elementary key intervals. We also analyse the distribution of the sets of meaningless terms in the lattice by selecting some sets as key vertices and study the cardinality in the intervals between key vertices. As an application, we prove that the lattice of meaningless sets is neither distributive nor modular. Interestingly, the example translates into a counterexample that the lattice of lambda theories is not modular.
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