The Casson invariant is an integer invariant of an oriented integral homology 3-sphere. It may be defined as an oriented count of conjugacy classes of irreducible representations
of its fundamental group into 
, with a normalization that assigns 0 to the 3-sphere.
Casson Invariant
See also
Fundamental GroupExplore with Wolfram|Alpha
References
Saveliev, N. Lectures on the Topology of 3-Manifolds: An Introduction to the Casson Invariant. Berlin, Germany: de Gruyter, 1999.Cite this as:
Weisstein, Eric W. "Casson Invariant." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/CassonInvariant.html