Narrow Results

This paper is concerned with bifurcation of limit cycles in a fourth-order near-Hamiltonian system with quartic perturbations. By bifurcation theory, proper perturbations are given to show that the system may have 20, 21 or 23 limit cycles with different distributions. This shows that H(4) ≥ 20, where H(n) is the Hilbert number for the second part of Hilbert's 16th problem. It is well known that H(2) ≥ 4, and it has been recently proved that H(3) ≥ 12. The number of limit cycles obtained in this paper greatly improves the best existing result, H(4) ≥ 15, for fourth-degree polynomial planar systems.

The bifurcations of generic heteroclinic loop with one nonhyperbolic equilibrium p₁ and one hyperbolic saddle p₂ are investigated, where p₁ is assumed to undergo transcritical bifurcation. Firstly, we discuss bifurcations of heteroclinic loop when transcritical bifurcation does not happen, the persistence of heteroclinic loop, the existence of homoclinic loop connecting p₁ (resp. p₂) and the coexistence of one homoclinic loop and one periodic orbit are established. Secondly, we analyze bifurcations of heteroclinic loop accompanied by transcritical bifurcation, namely, nonhyperbolic equilibrium p₁ splits into two hyperbolic saddles and , a heteroclinic loop connecting and p₂, homoclinic loop with (resp. p₂) and heteroclinic orbit joining and (resp. and p₂; p₂ and ) are found. The results achieved here can be extended to higher dimensional systems.

In the study of the perturbation of Hamiltonian systems, the first order Melnikov functions play an important role. By finding its zeros, we can find limit cycles. By analyzing its analytical property, we can find its zeros. The main purpose of this article is to summarize some methods to find its zeros near a Hamiltonian value corresponding to an elementary center, nilpotent center or a homoclinic or heteroclinic loop with hyperbolic saddles or nilpotent critical points through the asymptotic expansions of the Melnikov function at these values. We present a series of results on the limit cycle bifurcation by using the first coefficients of the asymptotic expansions.

In this paper, we study the number of limit cycles of some polynomial Liénard systems. Using the methods of the Melnikov functions and Hopf, homoclinic and heteroclinic bifurcation theory, we prove that H(2, 5) ≥ 4, H(6, 5) ≥ 7, H(10, 5) ≥ 11.

In this paper, we consider the problem of limit cycle bifurcation near center points and a Z₂-equivariant compound cycle in a polynomial Liénard system. Using the methods of Hopf, homoclinic and heteroclinic bifurcation theory, we found some new and better lower bounds of the maximal number of limit cycles for this system.

Bifurcations in a cubic system with a degenerate saddle point are investigated using the technique of blow-up, the method of planar perturbation theory and qualitative analysis. It has been found that after appropriate perturbations, at least 12 limit cycles can bifurcate from a degenerate saddle point in a type of cubic systems.

Like for smooth systems, a typical method to produce multiple limit cycles for a given piecewise smooth planar system is via homoclinic bifurcation. Previous works only focused on limit cycles that bifurcate from homoclinic orbits of piecewise-linear systems. In this paper, we consider for the first time the same problem for a class of general nonlinear piecewise smooth systems. By introducing the Dulac map in a small neighborhood of the hyperbolic saddle, we obtain the approximation of the Poincaré map for the nonsmooth homoclinic orbit. Then, we give conditions for the stability of the homoclinic orbit and conditions under which one or two limit cycles bifurcate from it. As an example, we construct a nonlinear piecewise smooth system with two limit cycles that bifurcate from a homoclinic orbit.

The bifurcation problems of twisted heteroclinic loop with two hyperbolic critical points are studied for the case ${\rho }_1^1>{\lambda }_1^1$, ${\rho }_2^1<{\lambda }_2^1$, ${\rho }_1^1{\rho }_2^1<{\lambda }_1^1{\lambda }_2^1$, where $-{\rho }_i^1<0$ and ${\lambda }_i^1>0$ are the pair of principal eigenvalues of unperturbed system at the critical point $p_i$, $i=1,2$. Under the transversal conditions, the authors obtained some results of the existence and the number of 1-homoclinic loop, 1-periodic orbit, double 1-periodic orbit, 2-homoclinic loop and 2-periodic orbit. Moreover, the relative bifurcation surfaces and the existence regions are given, and the corresponding bifurcation graphs are drawn.

This paper focuses on the number of limit cycles bifurcating from a symmetrical compound polycycle with three saddles. We use two methods, the Melnikov function method and the method of stability-changing of a homoclinic loop or a double homoclinic loop to study this problem. We find 15 limit cycles and 16 limit cycles respectively with four alien limit cycles under certain conditions.

For a class of three-dimensional piecewise linear systems with an admissible saddle-focus, the existence of three kinds of homoclinic loops is shown. Moreover, the birth and number of the periodic orbits induced by homoclinic bifurcation are investigated, and various sufficient conditions are obtained to guarantee the appearance of only one periodic orbit, finitely many periodic orbits or countably infinitely many periodic orbits. Furthermore, the stability of these newborn periodic orbits is analyzed in detail and some conclusions are made about them to be periodic saddle orbits or periodic sinks. Finally, some examples are given.