Conjecture on Hurwitz actions and reduced decompositions of braid Coxeter elements

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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:B↠Wp: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 Red⁡R(g)\operatorname{Red}_R(g) and Red⁡T(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 g∈Bg\in B such that: (1) the Hurwitz action is transitive on Red⁡R(g)\operatorname{Red}_R(g); (2) the Hurwitz action is transitive on Red⁡T(c)\operatorname{Red}_T(c); (3) the map pnp^n induces an isomorphism of BnB_n-sets from Red⁡R(g)\operatorname{Red}_R(g) to Red⁡T(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.

References

Primary source

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

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