The syntax of polytopal projections: From permutohedra to associahedra

Abstract

Tonks’ projection from the permutohedron to the associahedron and the Loday-Ronco map both send permutations to planar binary trees. We give a syntactic account of these maps in the equational calculus of the free non-symmetric, non-unital operad on one binary generator. The vertex restriction of Tonks’ projection is obtained by evaluating the head-insertion encoding on the reversed permutation, while the Loday-Ronco map is obtained by evaluating the decreasing encoding. We also give a local operadic proof that Tonks’ vertex map is order-preserving from the weak Bruhat order to the Tamari order.