Abstract:
In this article we study Hopf bifurcations and small amplitude limit cycles in a family of quadratic systems in the three dimensional space called Rucklidge systems. Bifurcation analysis at the equilibria of Rucklidge system is pushed forward toward the calculation of the second Lyapunov coefficient, which makes possible the determination of the Lyapunov and higher order structural stability.

Abstract:
The Hopf bifurcation characteristic in Langford system is analyzed and the control of Hopf bifurcation in this system is investigated by means of the method of linear state feedback control. The condition in which Hopf bifurcation occurs in the controlled system is arrived at in analysis and the analytical expressions of the limit cycles are obtained. Hopf bifurcation points are transformed and the stability of the limit cycles is controlled. The analytical results on controlling Hopf bifurcation points via the controllers agree well with the numerical results.

Abstract:
We present an unfolding of the codimension-two scenario of the simultaneous occurrence of a discontinuous bifurcation and an Andronov-Hopf bifurcation in a piecewise-smooth, continuous system of autonomous ordinary differential equations in the plane. We find the Hopf cycle undergoes a grazing bifurcation that may be very shortly followed by a saddle-node bifurcation of the orbit. We derive scaling laws for the bifurcation curves that emanate from the codimension-two bifurcation.

Abstract:
We introduce a versatile class of prototype dynamical systems for the study of complex bifurcation cascades of limit cycles, including bifurcations breaking spontaneously a symmetry of the system, period doubling bifurcations and transitions to chaos induced by sequences of limit cycle bifurcations. The prototype system consist of a $2d$-dimensional dynamical system with friction forces $f(V(\mathbf{x}))$ functionally dependent exclusively on the mechanical potential $V(\mathbf{x})$, which is typically characterized, here, by a finite number of local minima. We present examples for $d=1,2$ and simple polynomial friction forces $f(V)$, where the zeros of $f(V)$ regulate the relative importance of energy uptake and dissipation respectively, serving as bifurcation parameters. Starting from simple Hopf- and homoclinic bifurcations, complex sequences of limit cycle bifurcation are observed when energy uptake gains progressively in importance.

Abstract:
Lienard systems of the form $\ddot{x}+\epsilon f(x)\dot{x}+x=0$, with f(x) an even continous function, are considered. The bifurcation curves of limit cycles are calculated exactly in the weak ($\epsilon\to 0$) and in the strongly ($\epsilon\to\infty$) nonlinear regime in some examples. The number of limit cycles does not increase when $\epsilon$ increases from zero to infinity in all the cases analyzed.

Abstract:
This paper deals with the problem of location and existence of limit cycles for real planar polynomial differential systems. We provide a method to construct Poincar\'e--Bendixson regions by using transversal conics. We present several examples of known systems in the literature showing different features about limit cycles: hyperbolicity, Hopf bifurcation, sky-blue bifurcation, rotated vector fields, \ldots for which the obtained Poincar\'e--Bendixson region allows to locate the limit cycles. Our method gives bounds for the bifurcation values of parametrical families of planar vector fields and intervals of existence of limit cycles.

Abstract:
In this paper, we study the bifurcation of limit cycles in Lienard systems of the form dot(x)=y-F(x), dot(y)=-x, where F(x) is an odd polynomial that contains, in general, several free parameters. By using a method introduced in a previous paper, we obtain a sequence of algebraic approximations to the bifurcation sets, in the parameter space. Each algebraic approximation represents an exact lower bound to the bifurcation set. This sequence seems to converge to the exact bifurcation set of the system. The method is non perturbative. It is not necessary to have a small or a large parameter in order to obtain these results.

Abstract:
A zero-Hopf equilibrium is an isolated equilibrium point whose eigenvalues are $\pm \omega i\neq 0$ and $0$. In general for a such equilibrium there is no theory for knowing when from it bifurcates some small-amplitude limit cycle moving the parameters of the system. Here we study the zero-Hopf bifurcation using the averaging theory. We apply this theory to a Chua system depending on $6$ parameters, but the way followed for studying the zero-Hopf bifurcation can be applied to any other differential system in dimension $3$ or higher. In this paper first we show that there are three $4$-parameter families of Chua systems exhibiting a zero-Hopf equilibrium. After, by using the averaging theory, we provide sufficient conditions for the bifurcation of limit cycles from these families of zero-Hopf equilibria. From one family we can prove that $1$ limit cycle bifurcate, and from the other two families we can prove that $1$, $2$ or $3$ limit cycles bifurcate simultaneously.

Abstract:
Using Routh-Hurwitz criterion and Hopf bifurcation theorem, stability of the equilibrium points and Hopf bifurcation of coupled van der Pol and Duffing oscillators are investigated. The parametric curve corresponding to Hopf bifurcation and the existence region for the limit cycle in the parameter space are derived. The method of multiple time scales is used to deduce the analytical approximation of the limit cycle, and the accuracy of the analytical approximation is verified by direct numerical integration.

Abstract:
We present a detailed study of the effect of time delay on the collective dynamics of coupled limit cycle oscillators at Hopf bifurcation. For a simple model consisting of just two oscillators with a time delayed coupling, the bifurcation diagram obtained by numerical and analytical solutions shows significant changes in the stability boundaries of the amplitude death, phase locked and incoherent regions. A novel result is the occurrence of amplitude death even in the absence of a frequency mismatch between the two oscillators. Similar results are obtained for an array of N oscillators with a delayed mean field coupling and the regions of such amplitude death in the parameter space of the coupling strength and time delay are quantified. Some general analytic results for the N tending to infinity (thermodynamic) limit are also obtained and the implications of the time delay effects for physical applications are discussed.