The utmost rigidity property for quadratic foliations on $\mathbb{P}^2$ with an invariant line

TL;DR

This paper proves that for generic quadratic foliations on the projective plane with an invariant line, topological equivalence implies analytic equivalence, establishing the holonomy group's conjugacy class as a key invariant.

Contribution

It demonstrates that in the generic case, topological and analytic classifications coincide for quadratic foliations with an invariant line, refining previous results by Ilyashenko.

Findings

Topological equivalence implies analytic equivalence for generic quadratic foliations.

The holonomy group's conjugacy class serves as the modulus for classification.

Two generic quadratic vector fields that are orbitally topologically equivalent are affine equivalent.

Abstract

In this work we consider holomorphic foliations of degree two on the projective plane $\mathbb{P}^2$ having an invariant line. In a suitable choice of affine coordinates these foliations are induced by a quadratic vector field over the affine part in such a way that the invariant line corresponds to the line at infinity. We say that two such foliations are topologically equivalent provided there exists a homeomorphism of $\mathbb{P}^2$ which brings the leaves of one foliation onto the leaves of the other and preserves orientation both on the ambient space and on the leaves. The main result of this paper is that in the generic case two such foliations may be topologically equivalent if and only if they are analytically equivalent. In fact, it is shown that the analytic conjugacy class of the holonomy group of the invariant line is the modulus of both topological and analytic classification. We obtain as a corollary that two generic orbitally topologically equivalent quadratic vector fields on $\mathbb{C}^2$ must be affine equivalent. This result improves, in the case of quadratic foliations, a well-known result by Ilyashenko that claims that two generic and topologically equivalent foliations with an invariant line at infinity are affine equivalent provided they are close enough in the space of foliations and the linking homeomorphism is close enough to the identity map on $\mathbb{P}^2$.