Conjecture on cycles in the expression graph of a Coxeter group

Let (W,S)(W,S) be a Coxeter system, let wWw\in W, and let an expression for ww be a tuple (s1,s2,,sk)(s_1,s_2,\ldots,s_k) of elements of SS such that w=s1s2skw=s_1s_2\cdots s_k. Let E(w)\mathcal{E}(w) be the directed graph whose vertices are all expressions for ww, with directed edges given by braid moves. The graph R(w)\mathcal{R}(w) is the corresponding graph on reduced expressions.

Expression-graph conjecture. For every wWw\in W, the theorem of Bergeron, Ceballos and Labbé concerning cycles in R(w)\mathcal{R}(w) remains valid when R(w)\mathcal{R}(w) is replaced by E(w)\mathcal{E}(w).

This proposes extending the result from reduced to arbitrary, generally non-reduced, expressions. Unlike R(w)\mathcal{R}(w), the graph E(w)\mathcal{E}(w) is generally infinite and has several connected components, so the conjecture concerns cycles in a substantially larger graph.

Sources & referencesView supporting material

Primary source

Darij Grinberg and Alexander Postnikov, “Proof of a conjecture of Bergeron, Ceballos and Labbé”, arXiv:1603.03138 (2026).

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