Integrable system

In mathematics and physics, there are various distinct notions that are referred to under the name of integrable systems.

In the general theory of differential systems, there is Frobenius integrability, which refers to overdetermined systems. In the classical theory of Hamiltonian dynamical systems, there is the notion of Liouville integrability. This causes the trajectories to be fixed to smaller submanifolds allowing the solution to be expressed with a sequence of integrals (the origin of the name integrable). More generally, in differentiable dynamical systems integrability relates to the existence of foliations by invariant submanifolds within the phase space. Each of these notions involves an application of the idea of foliations, but they do not coincide. There are also notions of complete integrability, or exact solvability in the setting of quantum systems and statistical mechanical models. Integrability can often be traced back to the algebraic geometry of differential operators.

A differential system is said to be completely integrable in the Frobenius sense if the space on which it is defined has a foliation by maximal integral manifolds. The Frobenius theorem states that a system is completely integrable if and only if it generates an ideal that is closed under exterior differentiation. (See the article on integrability conditions for differential systems for a detailed discussion of foliations by maximal integral manifolds.)

In the context of differentiable dynamical systems, the notion of integrability refers to the existence of invariant, regular foliations; i.e., ones whose leaves are embedded submanifolds of the smallest possible dimension that are invariant under the flow. There is thus a variable notion of the degree of integrability, depending on the dimension of the leaves of the invariant foliation. This concept has a refinement in the case of Hamiltonian systems, known as complete integrability in the sense of Liouville (see below), which is what is most frequently referred to in this context.

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