# Commentarii Mathematici Helvetici

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**Volume 79, Issue 3, 2004, pp. 646**

**DOI: 10.1007/s00014-004-0816-y**

Published online: 2004-09-30

Rohlin's invariant and gauge theory, I. Homology 3-tori

Daniel Ruberman^{[1]}and Nikolai Saveliev

^{[2]}(1) Brandeis University, Waltham, USA

(2) University of Miami, Coral Gables, USA

This is the first in a series of papers exploring the relationship between the Rohlin invariant and gauge theory. We discuss a Casson-type invariant of a 3-manifold Y with the integral homology of the 3-torus, given by counting projectively flat U(2)-connections. We show that its mod 2 evaluation is given by the triple cup product in cohomology, and so it coincides with a certain sum of Rohlin invariants of Y. Our counting argument makes use of a natural action of H^1 (Y;Z_2) on the moduli space of projectively flat connections; along the way we construct perturbations that are equivariant with respect to this action. Combined with the Floer exact triangle, this gives a purely gauge-theoretic proof that Casson's homology sphere invariant reduces mod 2 to the Rohlin invariant.

*Keywords: *Casson invariant, Rohlin invariant, Floer homology, flat moduli spaces

Ruberman Daniel, Saveliev Nikolai: Rohlin's invariant and gauge theory, I. Homology 3-tori. *Comment. Math. Helv.* 79 (2004), 646-29. doi: 10.1007/s00014-004-0816-y