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Alexander Postnikov and Darij Grinberg
Proof of a conjecture of Bergeron, Ceballos and Labbé view print
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Published: |
November 4, 2017
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Keywords: |
Coxeter groups, reduced expressions, symmetric groups, spin symmetric groups, combinatorics, group theory |
Subject: |
20F55 |
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Abstract
The reduced expressions for a given element w of a Coxeter group
(W,S) can be regarded as the vertices of a directed graph
R(w); its arcs correspond to the braid moves.
Specifically, an arc goes from a reduced expression \overrightarrow{a} to a
reduced expression \overrightarrow{b} when \overrightarrow{b} is obtained
from \overrightarrow{a} by replacing a contiguous subword of the form
stst... (for some distinct s,t∈ S) by tsts... (where both
subwords have length ms,t, the order of st∈ W). We prove a strong
bipartiteness-type result for this graph R(w) : Not
only does every cycle of R(w) have even length;
actually, the arcs of R(w) can be colored (with
colors corresponding to the type of braid moves used), and to every color c
corresponds an opposite color
cop (corresponding to the reverses of the braid moves
with color c), and for any color c, the number of arcs in any given cycle
of R(w) having color in {c,cop} is even. This is a generalization and
strengthening of a 2014 result by Bergeron, Ceballos and Labbé.
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Author information
Alexander Postnikov:
Department of Mathematics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, United States
apost@math.mit.edu
Darij Grinberg:
Dunham Jackson Assistant Professor, School of Mathematics, University of Minnesota, 127 Vincent Hall, 206 Church St. SE, Minneapolis, MN 55455, USA
dgrinber@umn.edu
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