  In 1985 lectures at MSRI, A. Casson introduced an interesting integer valued
invariant for any oriented integral homology 3-sphere Y via beautiful
constructions on representation spaces (see [1] for an exposition). The Casson
invariant \lambda(Y) is roughly defined by measuring the oriented number of
irreducible representations of the fundamental group \pi_1(Y) in SU(2). Such an
invariant generalized the Rohlin invariant and gives surprising corollaries in
low dimensional topology.
