A Universal Bijection for Catalan Structures

Richard Brak

Published 2018-08-28Version 1

A Catalan magma is a unique factorisation normed magma with only one irreducible element. The \val partitions the base set into subsets enumerated by Catalan numbers. The primary theorem characterises the conditions which a set a with product map must satisfy in order to be a free magma generated by the irreducible elements. This theorem can be used to prove a set of objects (with a product map) is a Catalan magma. The isomorphism between Catalan magmas gives a "universal" bijection -- essentially one bijection algorithm for all pairs of families. The morphism property ensures the bijection is recursive. The universal bijection allows us to give some rigour to the idea of an "embedding" bijection between Catalan objects which, in many cases, shows how to embed an element of one Catalan family into one of a different family. Multiplication on the right (respectively left) by the generator gives rise to the right (respectively left) Narayana statistic. The statistic is invariant under the universal bijection and hence allows us to determine what structures of any Catalan family are associated with this refinement. We discuss the relation between the symbolic method for Catalan families and the magma structure on the base set defined by the symbolic method. This shows which "atomic" elements are also irreducible. The appendix gives the magma structure for 14 Catalan families.