arXiv subjects
Publications and source records attributed to J. L. Bravo.
Let $x'=S(t,x)$ be a differential equation in the cylinder, linear piecewise in $x$ and with trigonometric coefficients in $t$. In this paper, we provide an upper bound on the number of limit cycles in terms of the number of regions of the piecewise equation and the degree of the coefficients, that is, an analogue of Hilbert's 16th problem in this context.
The study of the dynamics of a continuous observable and non-controllable three-dimensional symmetric piecewise linear system with three zones can be reduced to the study of the existence of limit cycles for the piecewise differential equation $x'=ax+(b-a)\mathop{\rm sat}(x)+μ\sin t$, where $\operatorname{sat}$ stands for the normalized saturation function. This paper proves that the number of limit cycles of the equation is finite independently of $a,b,μ$. Moreover, it is proven that the maximum number of limit cycles is exactly one or three for certain values of the parameters. Using Melnikov theory for a certain deformation of the equation, it is proven that for small values of the perturbation parameter, there are exactly three, five or one limit cycles, depending on the values of $μ$. This strengthens the conjecture that the equation has at most five limit cycles.
We characterize global centers (all solutions are periodic) of the piecewise linear equation $x'=a(t)|x| + b(t)$ when the coefficients $a,b$ are trigonometric polynomials, under some generic hypotheses. We prove that the global centers are those determined by the composition condition on $a,b$. That is, the equation has a global center if and only if there exist polynomials $P, Q$ and a trigonometric polynomial $h$ such that $a(t)=P(h(t))h'(t)$, $b(t)=Q(h(t))h'(t)$.
In this paper, given two polynomials $f$ and $g$ of one variable and a $0$-cycle $C$ of $f$, we consider the deformation $f+εg$. We define two functions: the displacement function $Δ(t,ε)$ and its first order approximation: the abelian integral $M_1(t)$. The infinitesimal and tangential 16-th Hilbert problem for zero-cycles are problems of counting isolated regular zeros of $Δ(t,ε)$, for $ε$ small, or of $M_1(t)$, respectively. We show that the two problems are not equivalent and find optimal bounds, in function of the degrees of $f$ and $g$, for the infinitesimal and tangential 16-th Hilbert problem on zero-cycles. These two problems are the zero-dimensional analogue of the classical infinitesimal and tangential 16-th Hilbert problems for vector fields in the plane.
We consider the planar family of rigid systems of the form $x'=-y+xP(x,y), y'=x+yP(x,y)$, where $P$ is any polynomial with monomials of degree one and three. This is the simplest non-trivial family of rigid systems with no rotatory parameters. The family can be compactified to the Poincaré sphere such that the vector field along the equator is not identically null. We study the centers, singular points and limit cycles of that family on the plane and on the sphere.
For family $x'=(a_0+a_1\cos t+a_2 \sin t)|x|+b_0+b_1 \cos t+b_2 \sin t$, we solve three basic problems related with its dynamics. First, we characterize when it has a center (Poincaré center focus problem). Second, we show that each equation has a finite number of limit cycles (finiteness problem), and finally we give a uniform upper bound for the number of limit cycles (Hilbert's 16th problem).
A criterion is obtained for the semi-stability of the isolated singular positive closed solutions, i.e., singular positive limit cycles, of the Abel equation $x'=A(t)x^3+B(t)x^2$, where $A,B$ are smooth functions with two zeros in the interval $[0,T]$ and where these singular positive limit cycles satisfy certain conditions, which allows an upper bound on the number of limit cycles of the Abel equation to be obtained. The criterion is illustrated by obtaining an upper bound of two positive limit cycles for the family $A(t)=t(t-t_A)$, $B(t)=(t-t_B)(t-1)$, $t\in[0,1]$. In the linear trigonometric case, i.e., when $A(t)=a_0+a_1\sin t +a_2\cos t$, $B(t)=b_0+b_1\sin t+b_2 \cos t$, an upper bound of two limit cycles is also obtained for $a_0,b_0$ sufficiently small and in the region where two positive limit cycles bifurcate from the origin.
We study the rational solutions of the Abel equation $x'=A(t)x^3+B(t)x^2$ where $A,B\in C[t]$. We prove that if $deg(A)$ is even or $deg(B)>(deg(A)-1)/2$ then the equation has at most two rational solutions. For any other case, an upper bound on the number of rational solutions is obtained. Moreover, we prove that if there are more than $(deg(A)+1)/2$ rational solutions then the equation admits a Darboux first integral.
We study the analogue of the classical infinitesimal center problem in the plane, but for zero cycles. We define the displacement function in this context and prove that it is identically zero if and only if the deformation has a composition factor. That is, we prove that here the composition conjecture is true, in contrast with the tangential center problem on zero cycles. Finally, we give examples of applications of our results.