The combinator M and the Mockingbird lattice

Samuele Giraudo (1)

1 LIGM - Laboratoire d'Informatique Gaspard-Monge

We study combinatorial and order theoretic structures arising from the fragment of combinatory logic spanned by the basic combinator M. This basic combinator, named as the Mockingbird by Smullyan, is defined by the rewrite rule Mx 1 → x 1 x 1. We prove that the reflexive and transitive closure of this rewrite relation is a partial order on terms on M and that all connected components of its rewrite graph are Hasse diagram of lattices. This last result is based on the introduction of new lattices on duplicative forests, which are sorts of treelike structures. These lattices are not graded, not self-dual, and not semidistributive. We present some enumerative properties of these lattices like the enumeration of their elements, of the edges of their Hasse diagrams, and of their intervals. These results are derived from formal power series on terms and on duplicative forests endowed with particular operations.

Mots clés: Partial orders; Lattices; Combinatory logic; Rewrite systems; Treelike structures; Formal

Domaines: Combinatoire [math.CO]; Mathématique discrète [cs.DM]; Logique en informatique [cs.LO]

Samuele Giraudo. The combinator M and the Mockingbird lattice. Mathematical Structures in Computer Science, 2022, 32 (3), pp.271-299. ⟨10.1017/S0960129522000354⟩. ⟨hal-03669017v2⟩