Conjecture on Hurwitz actions and reduced decompositions of braid Coxeter elements

From papers

In either of the two settings, let BB be the associated braid group, let WW be the corresponding Coxeter or complex reflection group, let p:BWp:B\twoheadrightarrow W be the natural map, and let RR and TT be respectively the sets of braid reflections and reflections. Let nn be the rank, let BnB_n act by Hurwitz moves, and let RedR(g)\operatorname{Red}_R(g) and RedT(c)\operatorname{Red}_T(c) denote the reduced decompositions of gg and cc into elements of RR and TT. For a braid Coxeter element gg, write c:=p(g)c:=p(g), and let RgR_g and TcT_c be the reflections appearing in these reduced decompositions. The reduced-decomposition conjecture. There exists a braid Coxeter element gBg\in B such that: (1) the Hurwitz action is transitive on RedR(g)\operatorname{Red}_R(g); (2) the Hurwitz action is transitive on RedT(c)\operatorname{Red}_T(c); (3) the map pnp^n induces an isomorphism of BnB_n-sets from RedR(g)\operatorname{Red}_R(g) to RedT(c)\operatorname{Red}_T(c); and (4) the map pp induces a bijection from RgR_g to TcT_c. The conjecture is proved for the universal Coxeter group and braid group, and substantial cases are known for finite Coxeter groups and affine type An~\widetilde{A_n}; the general statement, including the complex-reflection-group setting, remains open.

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Sources & referencesView supporting material

Primary source

David Bessis, “A dual braid monoid for the free group”, arXiv:math/0401324 (2004).

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