Topology of real multi-affine hypersurfaces and a homological stability property

TL;DR

This paper establishes a sharp bound on the number of semi-algebraically connected components of real hypersurfaces defined by multi-affine polynomials, demonstrates exponential Betti number growth for certain symmetric hypersurfaces, and verifies a stability conjecture for symmetric real algebraic sets.

Contribution

The paper provides a new bound on the topology of multi-affine hypersurfaces, constructs examples with exponential Betti number growth, and confirms a conjecture on cohomology stability for symmetric algebraic sets.

Findings

Bound of 2^{d-1} on connected components, independent of dimension n.

Existence of hypersurfaces with Betti numbers growing exponentially with n.

Verification of the cohomological stability conjecture for a broader class of symmetric sets.

Abstract

Let $\mathrm{R}$ be a real closed field. We prove that the number of semi-algebraically connected components of a real hypersurface in $\mathrm{R}^n$ defined by a multi-affine polynomial of degree $d$ is bounded by $2^{d-1}$. This bound is sharp and is independent of $n$ (as opposed to the classical bound of $d(2d -1)^{n-1}$ on the Betti numbers of hypersurfaces defined by arbitrary polynomials of degree $d$ in $\mathrm{R}^n$ due to Petrovski{\u\i} and Ole{\u\i}nik, Thom and Milnor). Moreover, we show there exists $c > 1$, such that given a sequence $(B_n)_{n >0}$ where $B_n$ is a closed ball in $\mathrm{R}^n$ of positive radious, there exist hypersurfaces $(V_n)_{n_>0}$ defined by symmetric multi-affine polynomials of degree $4$, such that $\sum_{i \leq 5} b_i(V_n \cap B_n) > c^n$, where $b_i(\cdot)$ denotes the $i$-th Betti number with rational coeffcients. Finally, as an application of the main result of the paper we verify a representational stability conjecture due to Basu and Riener on the cohomology modules of symmetric real algebraic sets for a new and much larger class of symmetric real algebraic sets than known before.