Limit cycles in generalized Liénard systems

This paper presents some new results which we obtained recently for the study of limit cycles of nonlinear dynamical systems. Particular attention is given to small limit cycles of generalized Liénard systems in the vicinity of the origin. New results for a number of cases of the Liénard systems are presented with the Hilbert number, , for j = 4, i = 10, 11, 12, 13; j = 5, i = 6, 7, 8, 9; and j = 6, i = 5, 6. Detailed proofs for the existence of limit cycles are given in four cases.     

Periodic solutions of sublinear Liénard differential equations

In this paper, we study the existence of periodic solutions of the second order differential equations x^″+f(x)x^′+g(x)=e(t). Using continuation lemma, we obtain the existence of periodic solutions provided that F(x) () is sublinear when x tends to positive infinity and g(x) satisfies a new condition

where M, d are two positive constants.     

On the existence and uniqueness of limit cycles for generalized Liénard systems

In this paper, we consider a generalized Liénard system

     

A note on existence and uniqueness of limit cycles for Liénard systems

We consider the Liénard equation and we give a sufficient condition to ensure existence and uniqueness of limit cycles. We compare our result with some other existing ones and we give some applications.     

Limit cycles of a predator-prey model with intratrophic predation

We present some properties of a differential system that can be used to model intratrophic predation in simple predator-prey models. In particular, for the model we determine the maximum number of limit cycles that can exist around the only fine focus in the first quadrant and show that this critical point cannot be a centre.     

Estimating the number of limit cycles in polynomials systems

We describe a method based on algorithms of computational algebra for obtaining an upper bound for the number of limit cycles bifurcating from a center or a focus of polynomial vector field. We apply it to a cubic system depending on six parameters and prove that in the generic case at most six limit cycles can bifurcate from any center or focus at the origin of the system.     

The uniqueness of limit cycles for Liénard system

In monographs [Theory of Limit Cycles, 1984] and [Qualitative Theory of Differential Equations, 1985], eleven propositions by several mathematicians are listed on the uniqueness of limit cycles for equations of type (I), (II), and (III) of the quadratic ordinary differential systems. In this paper, we first point out that all these propositions were not completely proved since the equations under consideration do not satisfy the conditions of the theorems used to guarantee the uniqueness of limit cycles. Then we give a new set of theorems that guarantee the uniqueness of limit cycles for the Liénard systems, which not only can be applied to complete the proof of the propositions mentioned above but generalize many other uniqueness theorems as well. The conditions in these uniqueness theorems, which are independent and were obtained by different methods, can be combined into one improved general theorem that is easy to apply. Thus many of the most frequently used theorems on the uniqueness of limit cycles are corollaries of the results in this paper.     

Intersection with the vertical isocline in the generalized Liénard equations

We consider the generalized Liénard system

     

Multiple limit cycles for three dimensional Lotka-Volterra equations

A 3D competitive Lotka-Volterra equation with two limit cycles is constructed.     

The hyperelliptic limit cycles of the Liénard systems

For Liénard systems , with fm and gn real polynomials of degree m and n respectively, in [H. Zoladek, Algebraic invariant curves for the Liénard equation, Trans. Amer. Math. Soc. 350 (1998) 1681-1701] the author showed that if m?3 and m+1<n<2m there always exist Liénard systems which have a hyperelliptic limit cycle. Llibre and Zhang [J. Llibre, Xiang Zhang, On the algebraic limit cycles of Liénard systems, Nonlinearity 21 (2008) 2011-2022] proved that the Liénard systems with m=3 and n=5 have no hyperelliptic limit cycles and that there exist Liénard systems with m=4 and 5<n<8 which do have hyperelliptic limit cycles. So, it is still an open problem to characterize the Liénard systems which have an algebraic limit cycle in cases m>4 and m+1<n<2m. In this paper we will prove that there exist Liénard systems with m=5 and m+1<n<2m which have hyperelliptic limit cycles.     

Homoclinic orbits in generalized Liénard systems

This paper is devoted to the investigation on the existence of homoclinic orbits of the planar system of Liénard type , . Here h(y) is strictly increasing, but is not imposed h(±∞)=±∞. Sufficient conditions are given for a positive orbit of the system starting at a point on the curve h(y)=F(x) to approach the origin without intersecting the x-axis. The obtained theorems include previous results as special cases. Our results are applied to a concrete system and their sharpness are improved.     

Geometric tools to determine the hyperbolicity of limit cycles

In this paper we present a new method to study limit cycles' hyperbolicity. The main tool is the function ν=([V,W]∧V)/(V∧W), where V is the vector field under investigation and W a transversal one. Our approach gives a high degree of freedom for choosing operators to study the stability. It is related to the divergence test, but provides more information on the system's dynamics. We extend some previous results on hyperbolicity and apply our results to get limit cycles' uniqueness. Liénard systems and conservative + dissipative systems are considered among the applications.     

On the existence and uniqueness of limit cycles in planar continuous piecewise linear systems without symmetry

Some techniques to show the existence and uniqueness of limit cycles, typically stated for smooth vector fields, are extended to continuous piecewise-linear differential systems.New results are obtained for systems with three linearity zones without symmetry and having one equilibrium point in the central region. We also revisit the case of systems with only two linear zones giving shorter proofs of known results.A relevant application to the McKean piecewise linear model of a single neuron activity is included.     

Almost periodic solutions for a class of Liénard-type systems with multiple varying time delays

This paper considers the Liénard-type systems with multiple varying time delays. Some sufficient conditions for the existence and exponential stability of the almost periodic solutions are established, which are new and complement previously known results.     

Positive almost periodic solutions for a class of Liénard-type systems with multiple deviating arguments

This paper considers the Liénard-type systems with multiple deviating arguments. Sufficient conditions for the existence and exponential stability of positive almost periodic solutions are established, which are new and complement previously known results.     

Existence and non-existence of periodic solutions of generalized Lineard equations with damping of no lower bound

This paper establishes criteria for the existence and non-existence of nonzero periodic solutions of the generalized Liénard equationx +f(x,x)x +g(x)=0. The main goal is to study to what extent the dampingf can be small so as to guarantee the existence of nonzero periodic solutions of such a system. With some standard additional assumptions we prove that if for a small ¦x¦, ^± ¦f(x,y)¦^–1 dy=±, then the system has at least one nonzero periodic solution, otherwise, the system has no nonzero periodic solution. Many classical and well-known results can be proved as corollaries to ours.Supported by the National Natural Science Foundation of China.     

Liénard方程反周期解的存在性

利用度理论研究了二阶Liénard方程反周期解的存在性和二阶Duffing方程具有对称性的反周期解的存在性，改进了一些已有的结果．     

具有尖点的四次Liénard系统的极限环分支

该文研究了一类形如(x)=y,(y)=f(x)+εg(x)y的Liénard系统的Poincaré分支和Hopf分支,其中f(x)和g(x)分别是4次和3次多项式,证明了该系统绕原点最多能够产生3个极限环.