Recurrent point
of a dynamical system
A point of a dynamical system (also denoted by , see [2]) in a metric space that satisfies the following condition: For any there exists a such that all points of the trajectory are contained in an neighbourhood of any arc of time length of this trajectory (in other words, with any , the neighbourhood of the set
contains all of the trajectory ). In this case is called a recurrent trajectory.
Birkhoff's theorem: If the space is complete (e.g. ), then: 1) for a point to be recurrent it is necessary and sufficient that the closure of its trajectory be a compact minimal set; and 2) for the existence of a recurrent point it is sufficient that there be a point that is stable according to Lagrange (see Lagrange stability).
A recurrent point is stable according to Poisson (see Poisson stability), and if the space is complete, also stable according to Lagrange (see Lagrange stability). An almostperiodic (in particular, a fixed or periodic) point of a dynamical system is recurrent. In general, any point of a strictly ergodic dynamical system (in a complete space) is recurrent, but the restriction of a dynamical system to the closure of a recurrent trajectory (a minimal set) need not be a strictly ergodic dynamical system (Markov's example, see [2]).
References
[1]  G.D. Birkhoff, "Dynamical systems" , Amer. Math. Soc. (1927) 
[2]  V.V. Nemytskii, V.V. Stepanov, "Qualitative theory of differential equations" , Princeton Univ. Press (1960) (Translated from Russian) 
Comments
An almostperiodic point of a dynamical system on a metric space is a point with the following property: For every the set
is relatively dense in , i.e., there exists a length such that every interval in with length contains a point of . (Thus, one might say that the function is almostperiodic; cf. Almostperiod.)
Another important notion is that of an almostrecurrent point: A point such that for every the set
is relatively dense in , where is the open ball around . (This definition can easily be generalized to the case of a flow or a cascade on an arbitrary topological space.) The relationship of these notions with that of a recurrent point, Birkhoff's theorem and a number of other implications can be visualized as follows:
Figure: r080190a
Here the implication indicated by the dotted arrow holds only in a complete space, and denotes the closure of the trajectory of . The property "Sx is Lyapunov stable rel(ative) Sx" means that the family of functions from into is equicontinuous on (see Lyapunov stability). For a refinement of this diagram (including the notions of a pseudorecurrent point and a uniformly Poissonstable trajectory) see [a3].
In the literature on topological dynamics (in particular, in the literature directly or indirectly influenced by [a2]) another terminology is in use:'
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(To be precise, in [a1] "recurrent" means positive Poisson stable, i.e., belongs only to the limit set of its own trajectory.)
References
[a1]  H. Furstenberg, "Recurrence in ergodic theory and combinatorial number theory" , Princeton Univ. Press (1981) 
[a2]  W.H. Gottschalk, G.A. Hedlund, "Topological dynamics" , Amer. Math. Soc. (1955) 
[a3]  K.S. [K.S. Sibirskii] Sibirsky, "Introduction to topological dynamics" , Noordhoff (1975) (Translated from Russian) 
Recurrent point. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Recurrent_point&oldid=15436