219 research outputs found
Braids: A Survey
This article is about Artin's braid group and its role in knot theory. We set
ourselves two goals: (i) to provide enough of the essential background so that
our review would be accessible to graduate students, and (ii) to focus on those
parts of the subject in which major progress was made, or interesting new
proofs of known results were discovered, during the past 20 years. A central
theme that we try to develop is to show ways in which structure first
discovered in the braid groups generalizes to structure in Garside groups,
Artin groups and surface mapping class groups. However, the literature is
extensive, and for reasons of space our coverage necessarily omits many very
interesting developments. Open problems are noted and so-labelled, as we
encounter them.Comment: Final version, revised to take account of the comments of readers. A
review article, to appear in the Handbook of Knot Theory, edited by W.
Menasco and M. Thistlethwaite. 91 pages, 24 figure
Factoring in the hyperelliptic Torelli group
The hyperelliptic Torelli group is the subgroup of the mapping class group
consisting of elements that act trivially on the homology of the surface and
that also commute with some fixed hyperelliptic involution. The authors and
Putman proved that this group is generated by Dehn twists about separating
curves fixed by the hyperelliptic involution. In this paper, we introduce an
algorithmic approach to factoring a wide class of elements of the hyperelliptic
Torelli group into such Dehn twists, and apply our methods to several basic
elements.Comment: 9 pages, 7 figure
Commensurations of the Johnson kernel
Let K be the subgroup of the extended mapping class group, Mod(S), generated
by Dehn twists about separating curves. Assuming that S is a closed, orientable
surface of genus at least 4, we confirm a conjecture of Farb that Comm(K),
Aut(K) and Mod(S) are all isomorphic. More generally, we show that any
injection of a finite index subgroup of K into the Torelli group I of S is
induced by a homeomorphism. In particular, this proves that K is co-Hopfian and
is characteristic in I. Further, we recover the result of Farb and Ivanov that
any injection of a finite index subgroup of I into I is induced by a
homeomorphism. Our method is to reformulate these group theoretic statements in
terms of maps of curve complexes.Comment: Published by Geometry and Topology at
http://www.maths.warwick.ac.uk/gt/GTVol8/paper37.abs.htm
The level four braid group
By evaluating the Burau representation at t=-1, we obtain a symplectic
representation of the braid group. We study the resulting congruence subgroups
of the braid group, namely, the preimages of the principal congruence subgroups
of the symplectic group. Our main result is that the level 4 congruence
subgroup is equal to the group generated by squares of Dehn twists. We also
show that the image of the Brunnian subgroup of the braid group under the
symplectic representation is the level four congruence subgroup.Comment: 17 pages, 4 figures; minor corrections to the published versio
Evolution equations of curvature tensors along the hyperbolic geometric flow
We consider the hyperbolic geometric flow introduced by Kong and Liu [KL]. When the Riemannian
metric evolve, then so does its curvature. Using the techniques and ideas of
S.Brendle [Br,BS], we derive evolution equations for the Levi-Civita connection
and the curvature tensors along the hyperbolic geometric flow. The method and
results are computed and written in global tensor form, different from the
local normal coordinate method in [DKL1]. In addition, we further show that any
solution to the hyperbolic geometric flow that develops a singularity in finite
time has unbounded Ricci curvature.Comment: 15 page
Rotational symmetry of self-similar solutions to the Ricci flow
Let (M,g) be a three-dimensional steady gradient Ricci soliton which is
non-flat and \kappa-noncollapsed. We prove that (M,g) is isometric to the
Bryant soliton up to scaling. This solves a problem mentioned in Perelman's
first paper.Comment: Final version, to appear in Invent. Mat
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