Seifert Fibred Homology 3-Spheres and the Yang-Mills Equations on Riemann Surfaces with Marked Points
ADVANCES IN MATHEMATICS 96, 38-102 (1992) Seifert Fibred Homology 3-Spheres and the Yang-Mills Equations on Riemann Surfaces with Marked Points MIKIO FURUTA * University of Tokyo, Hongo Tokyo 113, Japan, and Mathematical Institute, University of Oxford, 24-29 St. Giles’, Oxford OX1 3LB, United Kingdom AND BRIAN STEER Hertford College and Mathematical Institute, University of Oxford, 24-29 St. Giles’, Oxford OXI 3LB, United Kingdom In [9], A. Floer considered the Chern-Simons functional on the space of connexions on a homology 3-sphere C and, using Morse theoretic methods, defined periodic homology groups HZ,(C). The Euler charac- teristic of the homology turns out to be twice Casson’s invariant [34] and the critical points of the functional itself were identified as the flat con- nexions on the trivial SU(2)-bundle over C. This paper was motivated by a wish to know what these groups are when C is a Seifert fibred homology sphere. Large steps towards realizing this were taken by E. B. Nasatyr [20] and, especially, by R. Fintushel and R. Stern in [8], where the indices of the critical manifolds (of the Chern-Simons functional) were calculated. These indices are all even and the manifolds themselves even-dimensional. Fintushel and Stern completed the computation when the critical manifolds were all either points or 2-spheres, so including all cases with 3 or 4 excep- tional orbits. They conjectured that in general the Floer (or instanton) homology groups were the direct sum, with the appropriate indices, of the homology of the critical manifolds. We show that this is so.
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