A-Discriminants for Complex Exponents, and Counting Real Isotopy Types

Abstract

We extend the definition of $\mathcal{A}$-discriminant varieties, and Kapranov's parametrization of $\mathcal{A}$-discriminant varieties, to complex exponents. As an application, we study the special case where $\mathcal{A}$ is a fixed real $n\times (n+3)$ matrix whose columns form the spectrum of an $n$-variate exponential sum $g$ with fixed sign vector for its coefficients: We prove that the number of possible isotopy types for the real zero set of $g$ is $O(n^2)$. The best previous upper bound was $2^{O(n^4)}$. Along the way, we also show that the singular loci of our generalized $\mathcal{A}$-discriminants are images of low-degree algebraic sets under certain analytic maps.