Critical loci and second-order singularities in arbitrary characteristic

Abstract

The critical loci of a map $f:X\to Y$ between smooth schemes over a field $k$ are the locally closed subschemes $\Sigma^i(f)\subseteq X$ where the differential of $f$ has constant rank. We prove that if $f : X\to \mathbb A^r$ is the general member of a suitably large linear family of maps from a smooth $k$-scheme $X$ to affine space, then the critical loci $\Sigma^i(f)$ are smooth, except in characteristic 2 where the first critical locus $\Sigma^1(f)$ may be singular at a finite set of points. Moreover, we compute the codimensions of the loci of second order singularities of such general maps $f :X \to \mathbb A^r$. In characteristics different from 2, the codimensions we find agree with those found by Levine in the context of differential topology. Finally, assuming that $k$ is an algebraically closed and $\dim X\ge \dim Y$, we give a local description of an arbitrary map $f :X \to Y$ at points of its first critical locus $\Sigma^1(f)$. In the case of functions and nondegenerate critical points, this description recovers the usual one from Morse theory.