Tamari lattices, forests and Thompson monoids

Abstract

A connection relating Tamari lattices on symmetric groups regarded as lattices under the weak Bruhat order to the positive monoid P of Thompson group F is presented. Tamari congruence classes correspond to classes of equivalent elements in P . The two well known normal forms in P correspond to endpoints of intervals in the weak Bruhat order that determine the Tamari classes. In the monoid P these correspond to the lexicographically largest and the lexicographically smallest form, while on the level of permutations they correspond to 132-avoiding and 231-avoiding permutations. Forests appear naturally in both contexts as they are used to model both permutations and elements of the Thompson monoid. The connection is then extended to Tamari orders on partitions of ( ( k − 1 ) n + 2 ) -gons into ( k + 1 ) -gons and Thompson monoids P k , k ≥ 2 .