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A Polygonal Perspective of Nielsen Reduction and the Chord Slide Groupoid

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(2010)cite arxiv:1010.5275 Comment: This survey (minus minor corrections) was written for and published in the abstracts for the 57th Topology Symposium which took place in Okayama, JP 2010. It summarizes the contents of a talk of the same title which was given by the author at that conference on 8/12/2010.

Zusammenfassung

Nielsen reduction is an algorithm which decomposes any automorphism of a free group into a product of elementary Nielsen transformations. While this may be applied to a mapping class of a surface $S_g,1$ with one boundary component, the resulting decomposition in general will not have a topological interpretation. In this survey, we discuss a variation called fatgraph Nielsen reduction which decomposes such a mapping class into elementary Nielsen transformations interpreted as rearrangements of polygon domains for $S_g,1$ described by systems of arcs in $S_g,1$. These elementary moves generate the chord slide groupoid of $S_g,1$, which we survey and describe in terms of generators and relations.

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