Counting indices of critical points of rank two of polynomial selfmaps of R4

For a generic f∈C∞(R4,R4) there is a discrete set Σ2(Df) of critical points of rank two, and there is an integer index Ip(Df) associated to any p∈Σ2(Df). We show how to compute ∑Ip(Df), where p∈Σ2(Df), in the case where f is a polynomial mapping.