Folheações algébricas projetivas

Abstract

An algebraic foliation of the projective plane over a field k can be given either by a vector field or a 1-form in P2, as dimension one and codimension one are the same notion since dim(P2) = 2. Then a natural question arises: How do vector fields and 1-forms in P2 relate? We will see that an 1-form ω is related with a vector field X belonging to the kernel of ω, that is, ω and X define the same foliation of the projective plane when ω(p)(X(p)) = 0 for all points p ∈P2. A second question concerns about the existence of algebraic curves that are invariant by a foliation of P2. Originally, Poincaré formulated the following problem: Is it possible to bound the degree of an invariant curve under a vector field in terms of the degree of the field? The problem has a negative answer, but by adding some hypothesis it can be reformulated in order to have a positive answer. If we assume that this invariant curve is smooth, we show that the degree of the curve is at most the degree of the vector field plus one. If an invariant curve is not smooth, we show that its degree can be limited in terms of regularity of its set of singularities.