Avoidance Loci of Real Projective Varieties
Elizabeth Pratt, Kexin Wang
TL;DR
Addresses the problem of characterizing real $k$-planes avoiding $X_{\mathbb{R}}$ for smooth totally real $X \subset \mathbb{P}^{n-1}$ by introducing the avoidance locus $\mathcal{A}_k(X)$ and relating its structure to higher Chow/Hurwitz forms; proves that $\mathcal{A}_k(X)$ is an open semi-algebraic union of regions in the complement of $\mathop{\mathrm{CH}}_{k+d-n+1}(X)$ and that distinct regions are non-adjacent. The paper develops computational tools for spaces, curves, and hypersurfaces, and connects to positivity theory via the Veronese model, establishing bounds on the number of components in terms of $X_{\mathbb{R}}$'s topology. It also introduces convexity notions (slice-convexity) and shows region-wise convexity in the hyperplane case. These results provide a structural bridge between real incidence geometry, discriminants, and polynomial positivity.
Abstract
We study real linear spaces in projective space that avoid the real points of a non-degenerate projective variety. For a variety $X \subset \mathbb{P}^{n-1}$ with a real smooth point, we define the avoidance locus $\mathcal{A}_k(X)$ as the subset of the real Grassmannian $\mathrm{Gr}(k,n)_{\mathbb{R}}$ consisting of linear spaces that meet $X$ transversely but contain no real point of $X$. Our construction generalizes the cone of positive polynomials on $\mathbb{R}^n.$ We prove that the avoidance locus is an open semi-algebraic set equal to a union of regions in the complement of a higher Chow form, and that distinct regions are non-adjacent. We present explicit examples for linear spaces, curves, and surfaces, and provide bounds on the number of connected components of $\mathcal{A}_{n-1}(X)$ in terms of the topology of the real locus $X_{\mathbb{R}}$. Finally, we prove that avoidance loci are slice-convex.