464 research outputs found
A short exposition of the Madsen-Weiss theorem
This is an exposition of a proof of the Madsen-Weiss Theorem, which asserts
that the homology of mapping class groups of surfaces, in a stable dimension
range, is isomorphic to the homology of a certain infinite loopspace that
arises naturally when one applies the "scanning method". The proof given here
utilizes simplifications introduced by Galatius and Randal-Williams.Comment: Version 2 adds three appendices containing background material: (1)
Gramain's proof of the Earle-Eells theorem on contractibility of the
components of diffeomorphism groups of surfaces, (2) the calculation of the
stable rational homology, and (3) a proof of the Group Completion Theorem
following an argument of Galatius. The exposition of the paper has also been
reorganized significantl
Homology stability for outer automorphism groups of free groups
We prove that the quotient map from Aut(F_n) to Out(F_n) induces an
isomorphism on homology in dimension i for n at least 2i+4. This corrects an
earlier proof by the first author and significantly improves the stability
range. In the course of the proof, we also prove homology stability for a
sequence of groups which are natural analogs of mapping class groups of
surfaces with punctures. In particular, this leads to a slight improvement on
the known stability range for Aut(F_n), showing that its i-th homology is
independent of n for n at least 2i+2.Comment: Published by Algebraic and Geometric Topology at
http://www.maths.warwick.ac.uk/agt/AGTVol4/agt-4-54.abs.htm
Erratum to: Homology stability for outer automorphism groups of free groups
We correct the proof of Theorem 5 of the paper Homology stability for outer
automorphism groups of free groups, by the first two authors.Comment: 7 pages, 3 figure
Stabilization for mapping class groups of 3-manifolds
We prove that the homology of the mapping class group of any 3-manifold
stabilizes under connected sum and boundary connected sum with an arbitrary
3-manifold when both manifolds are compact and orientable. The stabilization
also holds for the quotient group by twists along spheres and disks, and
includes as particular cases homological stability for symmetric automorphisms
of free groups, automorphisms of certain free products, and handlebody mapping
class groups. Our methods also apply to manifolds of other dimensions in the
case of stabilization by punctures.Comment: v4: improvements in the exposition as well as improvements in the
main combinatorial theorem in the paper, concerning complexes built from
join
Configuration spaces of rings and wickets
The main result in this paper is that the space of all smooth links in
Euclidean 3-space isotopic to the trivial link of n components has the same
homotopy type as its finite-dimensional subspace consisting of configurations
of n unlinked Euclidean circles (the "rings" in the title). There is also an
analogous result for spaces of arcs in upper half-space, with circles replaced
by semicircles (the "wickets" in the title). A key part of the proofs is a
procedure for greatly reducing the complexity of tangled configurations of
rings and wickets. This leads to simple methods for computing presentations for
the fundamental groups of these spaces of rings and wickets as well as various
interesting subspaces. The wicket spaces are also shown to be K(G,1)'s.Comment: 28 pages. Some revisions in the expositio
Finiteness of classifying spaces of relative diffeomorphism groups of 3-manifolds
The main theorem shows that if M is an irreducible compact connected
orientable 3-manifold with non-empty boundary, then the classifying space
BDiff(M rel dM) of the space of diffeomorphisms of M which restrict to the
identity map on boundary(M) has the homotopy type of a finite aspherical
CW-complex. This answers, for this class of manifolds, a question posed by M
Kontsevich. The main theorem follows from a more precise result, which asserts
that for these manifolds the mapping class group H(M rel dM) is built up as a
sequence of extensions of free abelian groups and subgroups of finite index in
relative mapping class groups of compact connected surfaces.Comment: 19 pages. Published copy, also available at
http://www.maths.warwick.ac.uk/gt/GTVol1/paper7.abs.htm
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