English

Isometry classes of generalized associahedra

Combinatorics 2010-03-31 v2

Abstract

Let (W,S)(W,S) be a finite Coxeter system acting by reflections on an R\mathbb R-Euclidean space with simple roots Δ={\assS}\Delta=\{\a_s | s\in S\} of the same length and fundamental weights Δ={vssS}\Delta^*=\{v_s | s\in S\}. We set M(e)=sSκsvsM(e)=\sum_{s\in S}\kappa_s v_s, κs>0\kappa_s>0, and for wWw\in W we set M(w)=w(M(e))M(w)=w(M(e)). The permutahedron Perm(W)Perm(W) is the convex hull of the set {M(w)wW}\{M(w) | w\in W\}. Given a Coxeter element cWc\in W, we have defined in a previous work a generalized associahedron Assoc(W)Asso_c(W) whose normal fan is the corresponding cc-Cambrian fan FcF_c defined by N. Reading. By construction, Assoc(W)Asso_c(W) is obtained from Perm(W)Perm(W) by removing some halfspaces according to a rule prescribed by cc. In this work, we classify the isometry classes of these realizations. More precisely, for (W,S)(W,S) an irreducible finite Coxeter system and c,cc,c' two Coxeter elements in WW, we have that Assoc(W)Asso_{c}(W) and Assoc(W)Asso_{c'}(W) are isometric if and only if μ(c)=c\mu(c') = c or μ(c)=w0c1w0\mu(c')=w_0c^{-1}w_0 for μ\mu an automorphism of the Coxeter graph of WW such that κs=κμ(s)\kappa_s=\kappa_{\mu(s)} for all sSs\in S. As a byproduct, we classify the isometric Cambrian fans of WW.

Keywords

Cite

@article{arxiv.0709.4421,
  title  = {Isometry classes of generalized associahedra},
  author = {Nantel Bergeron and Christophe Hohlweg and Carsten Lange and Hugh Thomas},
  journal= {arXiv preprint arXiv:0709.4421},
  year   = {2010}
}

Comments

12 pages, 4 figures, pdflatex: v2: correction of typos