The non-multiplicativity of the signature modulo 8 of a fibre bundle is an Arf-Kervaire invariant

Abstract

It was proved by Chern, Hirzebruch and Serre that the signature of a fibre bundle is multiplicative if the fundamental group of the base acts trivially on the cohomology ring of the fibre with real coefficients, in which case the signature of the total space equals the product of the signatures of base and fibre. Hambleton, Korzeniewski and Ranicki proved that in any case the signature is multiplicative modulo 4. In this paper we present two results concerning the multiplicativity modulo 8: firstly we identify the obstruction to multiplicativity modulo 8 with the Arf-Kervaire invariant of a Pontryagin squaring operation. Furthermore, we prove that if the fibre is even-dimensional and the action of the fundamental group of the base is trivial on the middle cohomology of the fibre with ${\mathbb{Z}}_4$ coefficients, then this Arf-Kervaire invariant takes value 0 and hence the signature is multiplicative modulo 8.

42 pages, 2 figures, final version to appear in AGT. Improved exposition, corrected typos and errors, but no changes in results with respect to version 1

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