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An ''isolated'' closed trajectory in the phase space of an [[Autonomous system|autonomous system]] of ordinary differential equations. A limit cycle corresponds to a periodic non-constant solution of the system. 

===Dynamics===
Limit cycles represent the simplest (after the steady states) type of behavior of a continuous time [[dynamical system]]. Theoretically all properties of limit cycles (their [[stability]] and [[bifurcation]]s) can be reduced to investigation of the associated [[Poincare first return map]]. In practice, however, the Taylor coefficients of the Poincare map can be obtained only in the form of integrals over the cycle, which may require some quite detailed knowledge of the shape of the cycle itself.

For instance, in the linear approximation if $\gamma:[0,T]\to\R^n$, $t\mapsto\gamma(t)$, is a limit cycle of period $T>0$ for the vector field $v(x)$ associated with the differential equation $\dot x=v(x)$, $x\in\R^n$, one obtains a linear (non-autonomous) system of differential equations
$$
 \dot z=A(t)z,\qquad z\in\R^n, \quad A(t)=\biggl(\frac{\partial v}{\partial x}(\gamma(t)\biggr),\ t\in [0,T].
$$
The corresponding ''Cauchy--Floquet'' linear operator $M\:\R^n\to\R^n$ maps a vector $a\in\R_n$ into the vector $Ma=z_a(T)$, where $z_a$ is the solution of the above system with the initial value $z_a(0)=a$. If this operator is [[hyperbolic]], i.e., has no modulus one eigenvalues ("[[characteristic exponent]]s"), then the stability pattern of the cycle (dimensions of the corresponding stable and unstable [[invariant manifolds]]) is completely determined (and coincides with that of the iterations $M^k$, $k\in\Z$).

===Limit cycles of planar vector fields===
On the two-dimensional sphere (and plane) the topological restrictions which forbid intersection of phase trajectories, make limit cycles the only possible limit motion not directly related to singular points (steady states, also known as stationary solutions). More precisely, if the $\Omega$-limit set of a non-periodic point $a\in \R^2$<ref>A closed invariant subset of the plane, defined as
$$
 \Omega(a)=\bigcap_{T<+\infty}\overline{\{g^t(a)|t\ge T\}},\qquad g^t(a)=\text{the flow map, }\left.\frac{\rd g^t(a)}{\rd t}\right|_{t=0}=v(a).
$$</ref>contains no singular point of the field $v$, then it must be a limit cycle ([[Poincare-Bendixson theory|Poincare-Bendixson]], 1886, 1901).

If the presence of singular points cannot be excluded, the situation becomes slightly more complicate. Under the assumption of analyticity one can show that the only possible limit sets for vector fields on the sphere are singular points, limit cycles and [[separatrix|separatrix polygons]], also known as [[polycycle]]s, which consist of singular points and connecting them arcs of [[separatrix|separatrices]].

For the same reasons [[bifurcation]]s of limit cycles, topological changes of the number of limit cycles, are possible only in annular neighborhoods of existing (multiple) cycles, singular points or polycycles.

===Hilbert 16th problem===
One of the most challenging problems which remains open for over 120 years, is the Hilbert's question on the number and position of limit cycles of a polynomial vector field on the plane (Problem 16, second part). Despite considerable progress in the last 25 years, the only known general result states that each polynomial vector field may have only finitely limit cycles (independently Yu. Ilyashenko and J. Ecalle, 1991). It is not known whether this number is uniformly bounded over all polynomial fields of degree $\le d$, even for $d=2$ (fields of degree $1$ cannot exhibit limit cycles at all).

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