Abstract
We compute the abelianisations of the mapping class groups of the manifolds \(W_g^{2n} = g(S^n \times S^n)\) for \(n \ge 3\) and \(g \ge 5\). The answer is a direct sum of two parts. The first part arises from the action of the mapping class group on the middle homology, and takes values in the abelianisation of the automorphism group of the middle homology. The second part arises from bordism classes of mapping tori and takes values in the quotient of the stable homotopy groups of spheres by a certain subgroup which in many cases agrees with the image of the stable J-homomorphism. We relate its calculation to a purely homotopy theoretic problem.
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Notes
Some authors denote the \((n-1)\)-connected cover of a space X by \(X\langle n \rangle \), and so write \(\mathbf {MO}\langle n \rangle \) for the Thom spectrum associated to the \((n-1)\)-connected cover of BO. We emphasise that our notation is different.
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Acknowledgments
The authors are grateful to Diarmuid Crowley for several useful discussions, both concerning attempts to prove Conjecture A, and especially concerning the extension problems discussed in Sect. 7. S. Galatius was partially supported by NSF grants DMS-1105058 and DMS-1405001, O. Randal-Williams was supported by the Herchel Smith Fund, and both authors were supported by ERC Advanced Grant No. 228082 and the Danish National Research Foundation through the Centre for Symmetry and Deformation.
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Galatius, S., Randal-Williams, O. Abelian quotients of mapping class groups of highly connected manifolds. Math. Ann. 365, 857–879 (2016). https://doi.org/10.1007/s00208-015-1300-2
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DOI: https://doi.org/10.1007/s00208-015-1300-2