Mapping class groups of compression bodies and 3-manifolds

Ulrich Oertel

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arXiv · math/0607444

Mapping class groups of compression bodies and 3-manifolds

Ulrich Oertel

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Abstract

We analyze the mapping class group of extendible automorphisms of the exterior boundary W of a compression body of dimension 3 or 4, which extend over the compression body (Q,V), where V is the interior boundary. Those that extend as automorphisms of (Q,V) rel V are called discrepant automorphisms, forming the mapping class group of discrepant automorphisms of W in Q. We describe a short exact sequence of these mapping class groups. For an orientable, compact, reducible 3 manifold W, there is a canonical "maximal" 4-dimensional compression body Q whose exterior boundary is W and whose interior boundary is the disjoint union of the irreducible summands of W. We obtain a short exact sequence for the mapping class group of a 3-manifold, which gives the mapping class group of the disjoint union of irreducible summands as a quotient of the entire mapping class group by the group of adjusting automorphisms. The group of discrepant automorphisms is described in terms of generators. The results are useful in a program for classifying automorphisms of compact 3-manifolds in the spirit of Nielsen-Thurston.

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BibTeX RIS

Ulrich Oertel. 2007-07-16. Mapping class groups of compression bodies and 3-manifolds. https://arxiv.org/abs/math/0607444

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