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

### Daniel Ruberman

Brandeis University, Waltham, USA### Nikolai Saveliev

University of Miami, Coral Gables, USA

## Abstract

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.

## Cite this article

Daniel Ruberman, Nikolai Saveliev, Rohlin's invariant and gauge theory, I. Homology 3-tori. Comment. Math. Helv. 79 (2004), no. 3, pp. 618–646

DOI 10.1007/S00014-004-0816-Y