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Solution to the Range Problem for Combinatory Logic

Intrigila B., Statman R.

Fundamenta Informaticae

2011

Vol. 111, nr 2

203-222

The lambda-theory H is obtained from beta-conversion by identifying all closed unsolvable terms (or, equivalently, termswithout head normal form). The range problemfor the theoryHasks whether a closed term has always (up to equality in H) either an infinite range or a singleton range (that is, it is a constant function). Here we give a solution to a natural version of this problem, giving a positive answer for the theory H restricted to Combinatory Logic. The method of proof applies also to the Lazy lambda-Calculus.

Two problems on reduction graphs in lambda calculus

Intrigila B., Laurenzi A.R.

Fundamenta Informaticae

2000

Vol. 44, Nr 1,2

133-144

In this paper we solve two open problems in the theory of reduction graphs in lambda calculus. The first question is whether the condensed reduction graph of a lambda term is always locally finite (conjecture of M.Venturini Zilli 1984). We give a negative answer to the conjecture by providing a counterexample. The second problem is whether there is a term such that its reduction graph is composed of two reduction cycles (not loops) intersecting in just one point (problem of J.W.Klop 1980). We show that such a term cannot exist.

A comprehensive setting for matching and unification over iterative terms

Intrigila B., Inverardi P., Zilli M.V.

Fundamenta Informaticae

1999

Vol. 39, Nr 3

273-304

Terms finitely representing infinite sequences of finite first-order terms have received attention by several authors. In this paper, we consider the class of recurrent terms proposed by H. Chen and J. Hsiang, and we extend it to allow infinite terms. This extension helps in clarifying the relationships between matching and unification over the class of terms we consider, that we call iterative terms. In fact, it holds that if a term s matches a term t by a substitution G, then the limit of iterations of the matching G, if it exists, is a most general unifier of s and t. A crucial feature of iterative terms is the notion of maximally-folded normal form that allows for a comprehensive treatment of both finite and infinite iterative terms. In this setting, infinite terms can be simply characterized as limits of sequences of finite terms. For finite terms we positively settle an open problem of H. Chen and J. Hsiang on the number of most general unifiers for a pair of terms.