We show that any equation dy/dx = P(x,y) with P a polynomial has a global (on ℝ²) smooth first integral nonconstant on any open domain. We also present an example of an equation without an analytic primitive first integral.

We study the continuous and discontinuous planar piecewise differential systems separated by a straight line and formed by an arbitrary linear system and a class of quadratic center. We show that when these piecewise differential systems are continuous, they can have at most one limit cycle. However, when the piecewise differential systems are discontinuous, we show that they can have at most two limit cycles, and that there exist such systems with two limit cycles. Therefore, in particular, we...

In this paper, we discuss the conditions for a center for the generalized Liénard system $$\frac{\mathrm{d}x}{\mathrm{d}t}=\varphi \left(y\right)-F\left(x\right),\frac{\mathrm{d}y}{\mathrm{d}t}=-g\left(x\right),$$ or $$\frac{\mathrm{d}x}{\mathrm{d}t}=\psi \left(y\right),\frac{\mathrm{dy}}{\mathrm{d}t}=-f\left(x\right)h\left(y\right)-g\left(x\right),$$ with $f\left(x\right)$, $g\left(x\right)$, $\varphi \left(y\right)$, $\psi \left(y\right)$, $h\left(y\right)ℝ\to ℝ$, $F\left(x\right)={\int }_0^{x}f\left(x\right)\mathrm{d}x$, and $xg\left(x\right)>0$ for $x\ne 0$. By using a different technique, that is, by introducing auxiliary systems and using the differential inquality theorem, we are able to generalize and improve some results in [1], [2].

In the paper, we give an existence theorem of periodic solution for Liénard equation $\dot{x}=y-F\left(x\right)$, $\dot{y}=-g\left(x\right)$. As a result, we estimate the amplitude $\rho \left(\mu \right)$ (maximal $x$-value) of the limit cycle of the van der Pol equation $\dot{x}=y-\mu (x^3/3-x)$, $\dot{y}=-x$ from above by $\rho \left(\mu \right)<2.3439$ for every $\mu \ne 0$. The result is an improvement of the author’s previous estimation $\rho \left(\mu \right)<2.5425$.

For an arbitrary analytic system which has a linear center at the origin we compute recursively all its Poincare-Lyapunov constants in terms of the coefficients of the system, giving an answer to the classical center problem. We also compute the coefficients of the Poincare series in terms of the same coefficients. The algorithm for these computations has an easy implementation. Our method does not need the computation of any definite or indefinite integral. We apply the algorithm to some polynomial...

In this paper the $\omega$-limit behaviour of trajectories of solutions of ordinary differential equations is studied by methods of an axiomatic theory of solution spaces. We prove, under very general assumptions, semi-invariance of $\omega$-limit sets and a Poincar’e-Bendixon type theorem.

We show the existence of a one-parameter family of cubic Kolmogorov system with an isochronous center in the realistic quadrant.

A sufficient condition for the nonoscillation of nonlinear systems of differential equations whose left-hand sides are given by $n$-th order differential operators which are composed of special nonlinear differential operators of the first order is established. Sufficient conditions for the oscillation of systems of two nonlinear second order differential equations are also presented.