EL-labelings and canonical spanning trees for subword complexes

TL;DR

This paper introduces EL-labelings for the increasing flip graph of subword complexes, enabling efficient facet generation and revealing combinatorial properties, with applications to Cambrian lattices and related structures.

Contribution

It provides a new EL-labeling framework for subword complexes, leading to canonical spanning trees, efficient algorithms, and deeper combinatorial insights.

Findings

Canonical spanning trees of the facet-ridge graph are constructed.

An efficient algorithm for generating all facets is developed.

EL-labelings reveal combinatorial properties of paths in the flip graph.

Abstract

We describe edge labelings of the increasing flip graph of a subword complex on a finite Coxeter group, and study applications thereof. On the one hand, we show that they provide canonical spanning trees of the facet-ridge graph of the subword complex, describe inductively these trees, and present their close relations to greedy facets. Searching these trees yields an efficient algorithm to generate all facets of the subword complex, which extends the greedy flip algorithm for pointed pseudotriangulations. On the other hand, when the increasing flip graph is a Hasse diagram, we show that the edge labeling is indeed an EL-labeling and derive further combinatorial properties of paths in the increasing flip graph. These results apply in particular to Cambrian lattices, in which case a similar EL-labeling was recently studied by M. Kallipoliti and H. M\"uhle.