Coincident root loci of binary forms

Coincident root loci are subvarieties of S d C 2 — the space of binary forms of degree d — labelled by partitions of d. Given a partition λ, let Xλ be the set of forms with root multiplicity corresponding to λ. There is a natural action of GL2(C) on S d C 2 and the coincident root loci are invariant under this action. We calculate their equivariant Poincaré duals generalizing formulas of Hilbert and Kirwan. In the second part we apply these results to present the cohomology ring of the corresponding moduli spaces (in the GIT sense) by geometrically defined relations. 1.