The Fintushel Stern formula asserts that the Casson invariant of a Brieskorn homology sphere 7( p, q, r) equals 1Â8 the signature of its Milnor fiber. We give a geometric proof of this formula, as opposite to computational methods used in the original proof. The formula is also refined to relate equivariant Casson invariants to equivariant signatures.
Academic PressKey Words: Milnor fiber; Casson invariant; knot signatures.Let 7( p, q, r) be the link of the singularity of f &1 (0) where f : C 3 Ä C is the polynomial f (x, y, z)=x p + y q +z r , and M( p, q, r) its Milnor fiber. Thus M( p, q, r) is a compact, simply connected, smooth parallelizable 4-manifold with boundary 7( p, q, r). We will assume that p, q, and r are pairwise relatively prime positive integers. Then 7( p, q, r) is an integral homology 3-sphere, and one may consider its Casson invariant, *(7( p, q, r)). In [8], while computing the Floer Homology of 7( p, q, r), Fintushel and Stern noticed the intriguing relation *(7( p, q, r))= where _(M( p, q, r)) stands for the signature of the manifold M( p, q, r).