Definition
A Casson invariant is a surjective map from oriented integral homology 3-spheres to satisfying the following properties:
- .
- Let be an integral homology 3-sphere. Then for any knot K and for any, the difference
is independent of n. Here denotes Dehn surgery on by K.
is equal to zero for any boundary link in .
The Casson invariant is unique up to sign.
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