Intersection number of a map with the set of matrices of positive corank

Abstract

The definition of the intersection number of a map with a closed manifold can be extended to the case of a closed stratified set such that the difference between dimensions of its two biggest strata is greater than $1$. The set Sigma of matrices of positive corank is an example of such a set. It turns out that the intersection number of a map from an (n-k+1)--dimensional manifold with boundary into the set of (n x k) real matrices with Sigma coincides with a homotopy invariant associated with a map going to the Stiefel manifold. In a polynomial case, we present an effective method to compute this intersection number. We also show how to use it to count the number mod 2 or the algebraic sum of cross--cap singularities of a map from an m--dimensional manifold with boundary to R^{2m-1}.