Nominal rewriting is based on the observation that if we add support for alphaequivalence to first-order syntax using the nominal-set approach, then systems with binding, including higher-order reduction schemes such as lambda-calculus betareduction, can be smoothly represented. Nominal rewriting maintains a strict distinction between variables of the objectlanguage (atoms) and of the meta-language (variables or unknowns). Atoms may be bound by a special abstraction operation, but variables cannot be bound, giving the framework a pronounced first-order character, since substitution of terms for variables is not capture-avoiding. We show how good properties of first-order rewriting survive the extension, by giving an efficient rewriting algorithm, a critical pair lemma, and a confluence theorem

Abstract. Nominal Logic is an extension of first-order logic with equality, name-binding, name-swapping, and freshness of names. Contrarily to higher-order logic, bound variables are treated as atoms, and only free variables are proper unknowns in nominal unification. This allows “variable capture”, breaking a fundamental principle of lambda-calculus. Despite this difference, nominal unification can be seen from a higher-order perspective. From this view, we show that nominal unification can be reduced to a particular fragment of higher-order unification problems: higher-order patterns unification. This reduction proves that nominal unification can be decided in quadratic deterministic time. 1

Abstract. Nominal Unification is an extension of first-order unification where terms can contain binders and unification is performed modulo α-equivalence. Here we prove that the existence of nominal unifiers can be decided in quadratic time. First, we linearly-reduce nominal unification problems to a sequence of freshness and equalities between atoms, modulo a permutation, using ideas as Paterson and Wegman for first-order unification. Second, we prove that solvability of these reduced problems may be checked in quadratic time. Finally, we point out how using ideas of Brown and Tarjan for unbalanced merging, we could solve these reduced problems more efficiently. 1.

Abstract. Nominal techniques were introduced to represent in a simple and natural way systems that involve binders. The syntax includes an abstraction operator and a primitive notion of name swapping. Nominal matching is matching modulo α-equality, and has applications in programming languages and theorem proving, amongst others. In this paper we describe efficient algorithms to check the validity of equations involving binders, and also to solve matching problems modulo α-equivalence, using the nominal approach.

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