# Limit set

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.

## Types

In general, limits sets can be very complicated as in the case of strange attractors, but for 2-dimensional dynamical systems the Poincaré–Bendixson theorem provides a simple characterization of all nonempty, compact -limit sets that contain at most finitely many fixed points as a fixed point, a periodic orbit, or a union of fixed points and homoclinic or heteroclinic orbits connecting those fixed points.

## Definition for iterated functions

Let be a metric space, and let be a continuous function. The -limit set of , denoted by , is the set of cluster points of the forward orbit of the iterated function . Hence, if and only if there is a strictly increasing sequence of natural numbers such that as . Another way to express this is

where denotes the *closure* of set . The closure is here needed, since we have not assumed that the underlying metric space of interest to be a complete metric space. The points in the limit set are non-wandering (but may not be **recurrent points**). This may also be formulated as the outer limit (limsup) of a sequence of sets, such that

If is a homeomorphism (that is, a bicontinuous bijection), then the -limit set is defined in a similar fashion, but for the backward orbit; *i.e.* .

Both sets are -invariant, and if is compact, they are compact and nonempty.

## Definition for flows

Given a real dynamical system (*T*, *X*, φ) with flow , a point *x* and an orbit γ through *x*, we call a point *y* an ω-**limit point** of *x* if there exists a sequence in **R** so that

- .

Analogously we call *y* an α-**limit point** of *x* if there exists a sequence in **R** so that

- .

The set of all ω-limit points (α-limit points) for a given orbit γ is called ω-**limit set** (α-**limit set**) for γ and denoted lim_{ω} γ (lim_{α} γ).

If the ω-limit set (α-limit set) is disjoint from the orbit γ, that is lim_{ω} γ ∩ γ = ∅ (lim_{α} γ ∩ γ = ∅), we call lim_{ω} γ (lim_{α} γ) a **ω-limit cycle** (**α-limit cycle**).

Alternatively the limit sets can be defined as

and

### Examples

- For any periodic orbit γ of a dynamical system, lim
_{ω}γ = lim_{α}γ = γ - For any fixed point of a dynamical system, lim
_{ω}= lim_{α}=

## References

- Teschl, Gerald (2012).
*Ordinary Differential Equations and Dynamical Systems*. Providence: American Mathematical Society. ISBN 978-0-8218-8328-0.

*This article incorporates material from Omega-limit set on PlanetMath, which is licensed under the Creative Commons Attribution/Share-Alike License.*