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Braid groups, mapping class groups and their homology with twisted coefficients

Published online by Cambridge University Press:  05 April 2021

ANDREA BIANCHI*
Affiliation:
Mathematical Institute of the University of Bonn, Endenicher Allee 60, 53115Bonn, Germany. e-mail: bianchi@math.uni-bonn.de Mathematical Institute of the University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark. e-mail: anbi@math.ku.dk

Abstract

We consider the Birman–Hilden inclusion $\phi\colon\Br_{2g+1}\to\Gamma_{g,1}$ of the braid group into the mapping class group of an orientable surface with boundary, and prove that $\phi$ is stably trivial in homology with twisted coefficients in the symplectic representation $H_1(\Sigma_{g,1})$ of the mapping class group; this generalises a result of Song and Tillmann regarding homology with constant coefficients. Furthermore we show that the stable homology of the braid group with coefficients in $\phi^*(H_1(\Sigma_{g,1}))$ has only 4-torsion.

Type
Research Article
Copyright
© The Author(s), 2021. Published by Cambridge University Press on behalf of Cambridge Philosophical Society

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