# Periodic point

*of a dynamical system*

A point on a trajectory of a periodic motion of a dynamical system ( or ) defined on a space , i.e. a point such that there is a number for which but for . This number is called the period of the point (sometimes, the name period is also given to all integer multiples of ).

The trajectory of a periodic point is called a closed trajectory or a loop. When the latter terms are used, one frequently abandons a concrete parametrization of the set of points on the trajectory with parameter and considers some class of equivalent parametrizations: If is a continuous action of the group on a topological space , a loop is considered as a circle that is topologically imbedded in ; if is a differentiable action of the group on a differentiable manifold , a loop is considered as a circle that is smoothly imbedded in .

If is a periodic point (and is a metric space), then the -limit set and the -limit set (cf. Limit set of a trajectory) coincide with its trajectory (as point sets). This property, to a certain extent, distinguishes a periodic point among all points that are not fixed, i.e. if the space in which the dynamical system is given is a complete metric space and if a point is such that , then is a fixed or a periodic point of .

#### References

[1] | V.V. Nemytskii, V.V. Stepanov, "Qualitative theory of differential equations" , Princeton Univ. Press (1960) (Translated from Russian) |

#### Comments

In arbitrary dynamical systems (where the phase space is not necessarily metric) the periodic points are characterized as follows (both for actions of and of ): A point is periodic if and only if its trajectory is a compact set consisting of more than one point. The question whether a given dynamical system has periodic points has been much studied. For dynamical systems on -manifolds, see e.g. [a4], [a6] and also Limit cycle; Poincaré–Bendixson theory and Kneser theorem. For Hamiltonian systems (cf. Hamiltonian system) see e.g. [a5], and for Hilbert's 16th problem (i.e., what is the number of limit cycles of a polynomial vector field in the plane?) see [a2]. Well-known is the Seifert conjecture. Every -dynamical system on has a periodic trajectory; see e.g. [a3]. For a connection between (the existence of) periodic trajectories and certain topological invariants (cf. also Singular point, index of a), see e.g. [a1].

#### References

[a1] | C. Conley, E. Zehnder, "Morse type index theory for flows and periodic solutions for Hamiltonian equations" Comm. Pure Appl. Math. , 37 (1984) pp. 207–253 |

[a2] | N.G. Lloyd, "Limit cycles of polynomial systems - some recent developments" T. Bedford (ed.) J. Swift (ed.) , New directions in dynamical systems , Cambridge Univ. Press (1988) pp. 192–234 |

[a3] | L. Markus, "Lectures in differentiable dynamics" , Amer. Math. Soc. (1980) pp. Appendix II |

[a4] | D.A. Neumann, "Existence of periodic orbits on 2-manifolds" J. Differential Eq. , 27 (1987) pp. 313–319 |

[a5] | P.H. Rabinowitz (ed.) A. Ambrosetti (ed.) I. Ekeland (ed.) E.J. Zehnder (ed.) , Periodic solutions of Hamiltonian systems and related topics , Proc. NATO Adv. Res. Workshop, 1986 , Reidel (1987) |

[a6] | R.J. Sacker, G.R. Sell, "On the existence of periodic solutions on 2-manifolds" J. Differential Eq. , 11 (1972) pp. 449–463 |

**How to Cite This Entry:**

Periodic point.

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