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Volume 191, No. 1 (1999), 49-74
R. Fenn, M.T. Greene, D. Rolfsen, C. Rourke and B. Wiest
Ordering the braid groups
Abstract:
We give an explicit geometric argument that Artin's braid group
Bn
is right-orderable. The construction is elementary, natural, and leads
to a new, effectively computable, canonical form for braids which we
call left-consistent canonical form. The left-consistent form
of a braid which is positive (respectively negative) in our order has
consistently positive (respectively negative) exponent in the smallest
braid generator which occurs. It follows that our ordering is
identical to that of Dehornoy (1995) constructed by very
different means, and we recover Dehornoy's main theorem that any braid
can be put into such a form using either positive or negative exponent
in the smallest generator but not both.
Our definition of order is strongly connected with Mosher's (1995)
normal form and this leads to an algorithm to decide
whether a given braid is positive, trivial, or negative which is
quadratic in the length of the braid word.
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