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In mathematics, especially in the study of dynamical systems, a limit set is the state a dynamical system reaches after an infinite amount of time has passed, by either going forward or backwards in time. Limit sets are important because they can be used to understand the long term behavior of a dynamical system. Examples of limit sets include fixed points, periodic orbits, limit cycles and attractors.
In general limits sets can be very complicated as in the case of strange attractors, but for 2-dimensional dynamical systems the Poincare-Bendixson theorem provides a simple characterization of all possible limit sets as a union of fixed points and periodic orbits.
Definition for iterated functions
Let X be a metric space, and let be a continuous function. The ?-limit set of , denoted by ?(x,f), is the set of cluster points of the forward orbit of the iterated function f. Hence, if and only if there is a strictly increasing sequence of natural numbers such that as . Another way to express this is

The points in the limit set are called recurrent points.
If f is a homeomorphism (that is, a bicontinuous bijection), then the a-limit set is defined in a similar fashion, but for the backward orbit; i.e. a(x,f) = ?(x,f - 1).
Both sets are f-invariant, and if X is compact, they are compact and nonempty.
Definition for flows
Given a real dynamical system (T, X, f) with flow , a point x and an orbit ? through x, we call a point y an ?-limit point of ? if there exists a sequence in R so that

.
Analogously we call y an a-limit point if there exists a sequence in R so that

.
The set of all ?-limit points (a-limit points) for a given orbit ? is called ?-limit set (a-limit set) for ? and denoted lim? ? (lima ?).
If the ?-limit set (a-limit set) is disjunct from the orbit ?, that is lim? ? n ? = Ø (lima ? n ? = Ø) , we call lim? ? (lima ?) a ?-limit cycle (a-limit cycle).
Alternatively the limit sets can be defined as

and

Examples
Properties
- lim? ? and lima ? are closed
- if X is compact then lim? ? and lima ? are nonempty, compact and simply connected
- lim? ? and lima ? are f-invariant, that is f(R × lim? ?) = lim? ? and f(R × lima ?) = lima ?
See also
This article incorporates material from Omega-limit set on PlanetMath, which is licensed under the GFDL.
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