Expected local topology of random complex submanifolds

TL;DR

This paper investigates the local topology of random complex submanifolds in Kähler manifolds, revealing asymptotic behavior of Betti numbers of their vanishing loci and contrasting with known models.

Contribution

It provides the first asymptotic formulas for the expected Betti numbers of local zero sets of random holomorphic sections in complex geometry.

Findings

Expected Betti number of the vanishing locus scales as d^n for large d.

Other Betti numbers grow slower than d^n, specifically o(d^n).

Results contrast with real algebraic and other smooth Gaussian models.

Abstract

Let $n\geq 2$ and $r\in \{1, \cdots, n-1\}$ be integers, $M$ be a compact smooth K\''ahler manifold of complex dimension $n$, $E$ be a holomorphic vector bundle with complex rank $r$ and equipped with an hermitian metric $h_E$, and $L$ be an ample holomorphic line bundle over $M$ equipped with a metric $h$ with positive curvature form. For any $d\in \mathbb{N}$ large enough, we endorse the space of holomorphic sections $H^0(M,E\otimes L^d)$ with the natural Gaussian measure associated to $h_E$ , $h$ and its curvature form. Let $U\subset M$ be an open subset with smooth boundary. We prove that the average of the $(n-r)$-th Betti number of the vanishing locus in $U$ of a random section $s$ of $H^0(M,E\otimes L^d)$ is asymptotic to ${n-1 \choose r-1} d^n\int_U c_1(L)^n$ for large $d$. On the other hand, the average of the other Betti numbers are $o(d^n)$. The first asymptotic recovers the classical deterministic global algebraic computation. Moreover, such a discrepancy in the order of growth of these averages is new and constrasts with all known other smooth Gaussian models, in particular the real algebraic one. We prove a similar result for the affine complex Bargmann-Fock model.