Some remarks about equations defining coincident root loci

Consider the projective variety $X_{\lambda }$ of binary forms of degree $d$ whose linear factors are distributed according to the partition $\lambda$ of $d$. We determine minimal sets of local generators of the fiber product of $X_{\lambda }$ with its normalization, and we show that the local Jacobian matrices of this product contain the product of the identity matrix of maximal rank with a unit. We use this to fill a gap in a crucial proof in Chipalkatti’s “On equations defining Coincident Root Loci”. Also, we give a new description of the singular locus of $X_{\lambda }$ and a criterion for the smoothness of $X_{\lambda }$.