In dynamical systems, a branch of mathematics, an invariant manifold is a topological manifold that is invariant under the action of the dynamical system.^[1] Examples include the slow manifold, center manifold, stable manifold, unstable manifold, subcenter manifold and inertial manifold.

Typically, although by no means always, invariant manifolds are constructed as a 'perturbation' of an invariant subspace about an equilibrium. In dissipative systems, an invariant manifold based upon the gravest, longest lasting modes forms an effective low-dimensional, reduced, model of the dynamics.^[2]

Definition

Consider the differential equation ${\displaystyle dx/dt=f(x),\ x\in \mathbb {R} ^{n},}$ with flow ${\displaystyle x(t)=\phi _{t}(x_{0})}$ being the solution of the differential equation with ${\displaystyle x(0)=x_{0}}$. A set ${\displaystyle S\subset \mathbb {R} ^{n}}$ is called an invariant set for the differential equation if, for each ${\displaystyle x_{0}\in S}$, the solution ${\displaystyle t\mapsto \phi _{t}(x_{0})}$, defined on its maximal interval of existence, has its image in ${\displaystyle S}$. Alternatively, the orbit passing through each ${\displaystyle x_{0}\in S}$ lies in ${\displaystyle S}$. In addition, ${\displaystyle S}$ is called an invariant manifold if ${\displaystyle S}$ is a manifold.^[3]

Examples

Invariant manifolds in non-autonomous dynamical systems

A differential equation

${\displaystyle {\frac {\mathrm {d} x}{\mathrm {d} t}}=f(x,t),\ x\in \mathbb {R} ^{n},\ t\in \mathbb {R} ,}$

represents a non-autonomous dynamical system, whose solutions are of the form ${\displaystyle x(t;t_{0},x_{0})=\phi _{t_{0}}^{t}(x_{0})}$ with ${\displaystyle x(t_{0};t_{0},x_{0})=x_{0}}$. In the extended phase space ${\displaystyle \mathbb {R} ^{n}\times \mathbb {R} }$ of such a system, any initial surface ${\displaystyle M_{0}\subset \mathbb {R} ^{n}}$ generates an invariant manifold

${\displaystyle {\mathcal {M}}=\cup _{t\in \mathbb {R} }\phi _{t_{0}}^{t}(M_{0}).}$

A fundamental question is then how one can locate, out of this large family of invariant manifolds, the ones that have the highest influence on the overall system dynamics. These most influential invariant manifolds in the extended phase space of a non-autonomous dynamical systems are known as Lagrangian Coherent Structures.^[4]

See also

• Hyperbolic set
• Lagrangian coherent structure
• Spectral submanifold

References