arXiv subjects

Fernando Sanz Sánchez

Publications and source records attributed to Fernando Sanz Sánchez.

9 recordsLinked to original sources

arXiv2024

Surfaces with central configuration and Dulac's problem for a three dimensional isolated Hopf singularity

Let $ξ$ be a real analytic vector field with an elementary isolated singularity at $0\in \mathbb{R}^3$ and eigenvalues $\pm bi,c$ with $b,c\in \mathbb{R}$ and $b\neq 0$. We prove that all cycles of $ξ$ in a sufficiently small neighborhood of $0$, if they exist, are contained in a finite number of subanalytic invariant surfaces entirely composed by a continuum of cycles. In particular, we solve Dulac's problem, i.e. finiteness of limit cycles, for such vector fields.

math.DS↗

arXiv2023

Trajectories of vector fields asymptotic to formal invariant curves

We prove that a formal curve $Γ$ that is invariant by a $C^\infty$ vector field $ξ$ of $\mathbb{R}^m$ has a geometrical realization, as soon as the Taylor expansion of $ξ$ is not identically zero along $Γ$. This means that there is a trajectory $γ$ of $ξ$ which is asymptotic to $Γ$. This result solves a natural question proposed by Bonckaert nearly forty years ago. We also construct an invariant $C^0$ manifold $S$ in some open horn around $Γ$ which is composed entirely of trajectories asymptotic to $Γ$, and contains the germ of any such trajectory. If $ξ$ is analytic, we prove that there exists a trajectory asymptotic to $Γ$ which is, moreover, non-oscillating with respect to subanalytic sets.

math.DS↗

arXiv2023

Stratification of three-dimensional real flows II: A generalization of Poincaré's planar sectorial decomposition

Let $ξ$ be an analytic vector field in $\mathbb{R}^3$ with an isolated singularity at the origin and having only hyperbolic singular points after a reduction of singularities $π:M\to\mathbb{R}^3$. Assuming certain conditions to be specified throughout the work at hand, we establish a theorem of stratification of the dynamics of $ξ$ that generalizes to dimension three the classical one, coming from Poincaré, about the decomposition of the dynamics of an analytic planar vector field into {\em parabolic}, {\em elliptic} or {\em hyperbolic} invariant sectors.

math.DS↗

arXiv2023

Turrittin's Theorem revisited. The real case

We establish a real version of Turrittin's result on polynomial and formal normal forms of linear systems of ODEs with meromorphic coefficients. Both the normal forms or the transformations used have only real coefficients. In order to adapt the proofs to the real case, we make a review of the result in the complex case.

math.CA↗

arXiv2022

Stratification of three-dimensional real flows I: Fitting Domains

Let $ξ$ be an analytic vector field in $\mathbb{R}^3$ with an isolated singularity at the origin and having only hyperbolic singular points after a reduction of singularities $π:M\to\mathbb{R}^3$. The union of the images by $π$ of the local invariant manifolds at those hyperbolic points, denoted by $Λ$, is composed of trajectories of $ξ$ accumulating to $0 \in \mathbb{R}^3$. Assuming that there are no cycles nor polycycles on the divisor of $π$, together with a Morse-Smale type property and a non-resonance condition on the eigenvalues at these points, in this paper we prove the existence of a fundamental system $\{V_n\}$ of neighborhoods well adapted for the description of the local dynamics of $ξ$: the frontier $Fr(V_n)$ is everywhere tangent to $ξ$ except around $Fr(V_n)\capΛ$, where transvesality is mandatory.

math.DS↗

arXiv2022

Solutions of definable ODEs with regular separation and dichotomy interlacement versus Hardy

We introduce a notion of regular separation for solutions of systems of ODEs $y'=F(x,y)$, where F is definable in a polynomially bounded o-minimal structure and $y = (y_1,y_2)$. Given a pair of solutions with flat contact, we prove that, if one of them has the property of regular separation, the pair is either interlaced or generates a Hardy field. We adapt this result to trajectories of three-dimensional vector fields with definable coefficients. In the particular case of real analytic vector fields, it improves the dichotomy interlaced/separated of certain integral pencils obtained by F. Cano, R. Moussu and the third author. In this context, we show that the set of trajectories with the regular separation property and asymptotic to a formal invariant curve is never empty and it is represented by a subanalytic set of minimal dimension containing the curve. Finally, we show how to construct examples of formal invariant curves which are transcendental with respect to subanalytic sets, using the so-called (SAT) property introduced by J.-P. Rolin, R. Shaefke and the third author.

math.DS↗

arXiv2022

Stable manifolds of biholomorphisms in $\mathbb{C}^n$ asymptotic to formal curves

Given a germ of biholomorphism $F\in\mathrm{Diff}(\mathbb{C}^n,0)$ with a formal invariant curve $Γ$ such that the multiplier of the restricted formal diffeomorphism $F|_Γ$ is a root of unity or satisfies $|(F|_Γ)'(0)|<1$, we prove that either $Γ$ is contained in the set of periodic points of $F$ or there exists a finite family of stable manifolds of $F$ where all the orbits are asymptotic to $Γ$ and whose union eventually contains every orbit asymptotic to $Γ$. This result generalizes to the case where $Γ$ is a formal periodic curve.

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arXiv2018

Real Analytic Vector Fields with First Integral and Separatrices

We prove that a germ of analytic vector field at $(\mathbb{R}^3,0)$ that possesses a non-constant analytic first integral has a real formal separatrix. We provide an example which shows that such a vector field does not necessarily have a real analytic separatrix.

math.DS↗

arXiv2014

Parabolic curves of diffeomorphisms asymptotic to formal invariant curves

We prove that if $F$ is a tangent to the identity diffeomorphism at $0\in\mathbb{C}^2$ and $Γ$ is a formal invariant curve of $F$ then there exists a parabolic curve (attracting or repelling) of $F$ asymptotic to $Γ$. The result is a consequence of a more general one in arbitrary dimension, where we prove the existence of parabolic curves of a tangent to the identity diffeomorphism $F$ at $0\in\mathbb{C}^n$ asymptotic to a given formal invariant curve under some additional conditions, expressed in terms of a reduction of $F$ to a special normal form by means of blow-ups and ramifications along the formal curve.

math.DS↗