Compatibility fans for graphical nested complexes

TL;DR

This paper introduces a new class of compatibility fans for graphical nested complexes, generalizing known constructions and providing alternative fan realizations that support the nested complex structure.

Contribution

It defines a compatibility degree for tubes in a graph and proves that the resulting vectors support a complete simplicial fan for any maximal tubing.

Findings

Supports a complete simplicial fan realizing the nested complex

Recovers known fan realizations for path graphs

Generalizes compatibility fans to arbitrary graphs

Abstract

Graph associahedra are natural generalizations of the classical associahedra. They provide polytopal realizations of the nested complex of a graph $G$, defined as the simplicial complex whose vertices are the tubes (i.e. connected induced subgraphs) of $G$ and whose faces are the tubings (i.e. collections of pairwise nested or non-adjacent tubes) of $G$. The constructions of M. Carr and S. Devadoss, of A. Postnikov, and of A. Zelevinsky for graph associahedra are all based on the nested fan which coarsens the normal fan of the permutahedron. In view of the combinatorial and geometric variety of simplicial fan realizations of the classical associahedra, it is tempting to search for alternative fans realizing graphical nested complexes. Motivated by the analogy between finite type cluster complexes and graphical nested complexes, we transpose in this paper S. Fomin and A. Zelevinsky's construction of compatibility fans from the former to the latter setting. For this, we define a compatibility degree between two tubes of a graph $G$. Our main result asserts that the compatibility vectors of all tubes of $G$ with respect to an arbitrary maximal tubing on $G$ support a complete simplicial fan realizing the nested complex of $G$. In particular, when the graph $G$ is reduced to a path, our compatibility degree lies in $\{-1,0,1\}$ and we recover F. Santos' Catalan many simplicial fan realizations of the associahedron.