Parameterwise Sharpness of Khovanskii's Bezout-type Bound for Pfaffian Functions

Abstract

Khovanskii's theorem gives a Bezout-type upper bound for the number of isolated real solutions of a system of $n$ Pfaffian equations in $n$ variables in terms of three complexity parameters: the chain-degree $\hat{I}\pm$, the degrees ${\hat{I}}_i^2$ of the Pfaffian functions, and the order $s$ of the underlying Pfaffian chain. Despite its fundamental role in Pfaffian geometry and o-minimality, little is known about the sharpness of this bound. We investigate the theorem from a parameter-by-parameter perspective. We show that its dependence on the chain-degree $\hat{I}\pm$ is asymptotically sharp by constructing, for every $\hat{I}\pm ,s\in \mathbb{N}$, a Pfaffian function of format $(\hat{I}\pm ,1,s)$ with at least $\hat{I}{\pm }^s$ nondegenerate real zeros. We also show that its dependence on the degrees ${\hat{I}}_i^2$ is asymptotically sharp: for fixed $n$ and $s$, we construct Pfaffian systems having $Ω_{n,s}(β^{n+s})$ regular common zeros, matching the order of growth predicted by Khovanskii's theorem as ${\hat{I}}^2\to \infty$.

v1: 22 pages, 1 figure v2: Tightened exposition, and small change to title