TITLE: Casson invariant and Euler characteristic
QUESTION [5 upvotes]: A slogan I frequently hear is: "the Casson invariant is the Euler characteristic of the Floer homology of flat SU(2)-connections on the integral homology sphere". Is there a single paper/reference that essentially states this as a result? It has been difficult to locate precise definitions. In addition, a sketch of how this works would be very helpful.

REPLY [6 votes]: Just to finalize comments since people are upvoting the question: The canonical reference is Taubes' "Casson's invariant and gauge theory" which makes the statement rigorous and has all the definitions (based on the relevant Chern-Simons functional). Floer's paper "An instanton-invariant for 3-manifolds" does build the Floer homology using this functional, and asserts that its Euler characteristic is (twice) the Casson invariant, but he only quotes Taubes' paper (that's what the reference [3] in Floer's paper really is).
Although I think Taubes' paper is beautiful enough that a sketch isn't needed, you can find a background sketch/summary in Saveliev's book ("Invariants of homology 3-spheres"), Chapter 5: "Casson Invariant and Gauge Theory".