DCDS-B
On the limit cycles of the Floquet differential equation
Jaume Llibre Ana Rodrigues
We provide sufficient conditions for the existence of limit cycles for the Floquet differential equations $\dot {\bf x}(t) = A{\bf x}(t)+ε(B(t){\bf x}(t)+b(t))$, where ${\bf x}(t)$ and $b(t)$ are column vectors of length $n$, $A$ and $B(t)$ are $n\times n$ matrices, the components of $b(t)$ and $B(t)$ are $T$--periodic functions, the differential equation $\dot {\bf x}(t)= A{\bf x}(t)$ has a plane filled with $T$--periodic orbits, and $ε$ is a small parameter. The proof of this result is based on averaging theory but only uses linear algebra.
keywords: Floquet differential equation averaging theory. periodic solution limit cycle
DCDS
Hopf bifurcation for some analytic differential systems in $\R^3$ via averaging theory
Jaume Llibre Clàudia Valls
We study the Hopf bifurcation from the singular point with eigenvalues $a$ε$ \ \pm\ bi$ and $c $ε located at the origen of an analytic differential system of the form $ \dot x= f( x)$, where $x \in \R^3$. Under convenient assumptions we prove that the Hopf bifurcation can produce $1$, $2$ or $3$ limit cycles. We also characterize the stability of these limit cycles. The main tool for proving these results is the averaging theory of first and second order.
keywords: Hopf bifurcation averaging theory differential systems in dimension 3.
CPAA
Centers for polynomial vector fields of arbitrary degree
Jaume Llibre Claudia Valls
We present two new families of polynomial differential systems of arbitrary degree with centers, a two--parameter family and a four--parameter family.
keywords: polynomial vector fields arbitrary degree Centers
DCDS
The cyclicity of period annuli of some classes of reversible quadratic systems
G. Chen C. Li C. Liu Jaume Llibre
The cyclicity of period annuli of some classes of reversible and non-Hamiltonian quadratic systems under quadratic perturbations are studied. The argument principle method and the centroid curve method are combined to prove that the related Abelian integral has at most two zeros.
keywords: reversible systems Limit cycle Abelian integral.
DCDS-B
Zero-Hopf bifurcation for a class of Lorenz-type systems
Jaume Llibre Ernesto Pérez-Chavela
In this paper we apply the averaging theory to a class of three-dimensional autonomous quadratic polynomial differential systems of Lorenz-type, to show the existence of limit cycles bifurcating from a degenerate zero-Hopf equilibrium.
keywords: Three-dimensional differential systems Lorenz-type system. limit cycles zero-Hopf equilibrium
DCDS
Periodic solutions of El Niño model through the Vallis differential system
Rodrigo Donizete Euzébio Jaume Llibre
By rescaling the variables, the parameters and the periodic function of the Vallis differential system we provide sufficient conditions for the existence of periodic solutions and we also characterize their kind of stability. The results are obtained using averaging theory.
keywords: El Niño model. Vallis system periodic solutions
DCDS
Regularization of discontinuous vector fields in dimension three
Jaume Llibre Marco Antonio Teixeira
In this paper vector fields around the origin in dimension three which are approximations of discontinuous ones are studied. In a former work of Sotomayor and Teixeira [6] it is shown, via regularization, that Filippov's conditions are the natural ones to extend the orbit solutions through the discontinuity set for vector fields in dimension two. In this paper we show that this is also the case for discontinuous vector fields in dimension three. Moreover, we analyse the qualitative dynamics of the local flow in a neighborhood of the codimension zero regular and singular points of the discontinuity surface.
keywords: Regularization and discontinuous vector fields.
DCDS
Center conditions for a class of planar rigid polynomial differential systems
Jaume Llibre Roland Rabanal
In general the center--focus problem cannot be solved, but in the case that the singularity has purely imaginary eigenvalues there are algorithms to solving it. The present paper implements one of these algorithms for the polynomial differential systems of the form \[ \dot x= -y + x f(x) g(y),\quad \dot y= x+y f(x) g(y), \] where $f(x)$ and $g(y)$ are arbitrary polynomials. These differential systems have constant angular speed and are also called rigid systems. More precisely, in this paper we give the center conditions for these systems, i.e. the necessary and sufficient conditions in order that they have an uniform isochronous center. In particular, the existence of a focus with the highest order is also studied.
keywords: limit cycles the center problem Isochronous centers Lyapunov quantities. focal basis
DCDS
On the existence of bi--pyramidal central configurations of the $n+2$--body problem with an $n$--gon base
Montserrat Corbera Jaume Llibre
In this paper we prove the existence of central configurations of the $n+2$--body problem where $n$ equal masses are located at the vertices of a regular $n$--gon and the remaining $2$ masses, which are not necessarily equal, are located on the straight line orthogonal to the plane containing the $n$--gon passing through its center. Here this kind of central configurations is called bi--pyramidal central configurations. In particular, we prove that if the masses $m_{n+1}$ and $m_{n+2}$ and their positions satisfy convenient relations, then the configuration is central. We give explicitly those relations.
keywords: $n+2$-body problem bi-pyramidal central configurations. Spatial central configurations
DCDS-B
Brief survey on the topological entropy
Jaume Llibre
In this paper we give a brief view on the topological entropy. The results here presented are well known to the people working in the area, so this survey is mainly for non--experts in the field.
keywords: graph map interval map compact manifold. Topological entropy

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