THE SL2(ℂ) CASSON INVARIANT FOR SEIFERT FIBERED HOMOLOGY SPHERES AND SURGERIES ON TWIST KNOTS

We derive a simple closed formula for the SL 2 (ℂ) Casson invariant for Seifert fibered homology 3-spheres using the correspondence between SL 2 (ℂ) character varieties and moduli spaces of parabolic Higgs bundles of rank two. These results are then used to deduce the invariant for Dehn surgeries on twist knots by combining computations of the Culler-Shalen norms with the surgery formula for the SL 2 (ℂ) Casson invariant.