Dyer–Hohlweg bijection conjecture for low elements and small inversion sets

Let (W,S)(W,S) be a Coxeter system with root system cPhicPhi, positive roots cPhi+cPhi^+, and inversion set N(w)N(w) for weqthinspacecinWw eqthinspacecin W. A positive root aeqthinspacecincPhi+a eqthinspacecincPhi^+ is small if, for every beqthinspacecincPhi+{a}b eqthinspacecincPhi^+\setminus\{a\}, there \exists weqthinspacecinWw eqthinspacecin W such that aeqthinspacecinN(w)a eqthinspacecin N(w) and botinN(w)b otin N(w). Let cSigmacSigma be the set of small roots, let cSigma(w)=cSigmaN(w)cSigma(w)=cSigma\cap N(w), and let cLambdacLambda be the set of all small inversion sets. An element weqthinspacecinWw eqthinspacecin W is low if

N(w)=coperatornamecone(cSigma(w))\capcPhi+.N(w)=coperatorname{cone}(cSigma(w))\capcPhi^+.

Let LL be the set of low elements. Dyer–Hohlweg's conjecture. The map

wcSigma(w):L\longrightarrowcLambdaw\longmapsto cSigma(w):L\longrightarrowcLambda

is a bijection between low elements and small inversion sets. This conjecture asks whether every small inversion set is realized by exactly one low element. The map is already known to be injective because inversion sets determine elements of the Coxeter group; the conjecture concerns surjectivity and its connection with the geometry of Shi regions.

Sources & referencesView supporting material

Primary source

Balthazar Charles, “Low elements and small inversion sets are in bijection in rank 3 Coxeter groups”, arXiv:2104.03040 (2021).

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