Densely ordered braid subgroups
Abstract
Dehornoy showed that the Artin braid groups Bn are left-orderable. This ordering is discrete, but we show that, for n >2 the Dehornoy ordering, when restricted to certain natural subgroups, becomes a dense ordering. Among subgroups which arise are the commutator subgroup and the kernel of the Burau representation (for those n for which the kernel is nontrivial). These results follow from a characterization of least positive elements of any normal subgroup of Bn which is discretely ordered by the Dehornoy ordering.
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