المستخلص: |
The problem of existence of periodic orbits and critical points is fundamental in the analysis of behaviour of differential equations and in several applications. However, in many cases it is not easy to find such a solution. Two-dimensional systems play here an important role. One of reasons is that a one-dimensional equation of the second order may be reduced to a system of two equations of the first order. The Poincaré-Bendixson Theorem gives conditions which enable us to prove the existence of a periodic solution of the equation. Moreover, for two-dimensional systems in many cases the existence of a periodic orbit gives also the existence of a critical point. The existence of a particular kind of periodic orbits, i.e. limit cycles, is in many situations particularly interesting. Frequently, it follows from the Poincaré– Bendixson Theorem.
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