# Integral manifold

A set of points of the phase space (-space) of the system

(*) |

filled by the integral curves of this system (cf. Integral curve), defined for all and forming a manifold in -space. The dimension of the section of by the plane is usually called the dimension of the integral manifold . In the definition of an integral manifold, the requirement that it be a manifold is sometimes replaced by the requirement that the set be representable analytically by an equation

with a function defined for all in and in some domain and possessing a specific smoothness in , for . The integral manifold is then called -dimensional and of the same smoothness as the function .

Examples. An integral curve of a periodic solution of the system (*), that is, a periodic integral curve; the family of integral curves of the system (*) formed by a family of quasi-periodic solutions of (*), filling an -dimensional torus in the -space when , that is, an -dimensional toroidal integral manifold; etc.

The integral manifolds that have been most extensively studied are the toroidal manifolds, that is, sets that are tori for any fixed . These manifolds are widely encountered in systems of type (*) describing oscillatory processes.

#### References

[1] | N.N. Bogolyubov, "On certain statistical methods in mathematical physics" , L'vov (1945) (In Russian) |

[2] | N.N. Bogolyubov, Yu.A. Mitropol'skii, "The method of integral manifolds in non-linear mechanics" , Proc. Internat. Symp. Non-linear Oscillations , 1 , Kiev (1963) pp. 96–154 (In Russian) |

[3] | N.N. Bogolyubov, Yu.A. Mitropol'skii, "The method of integral manifolds in the theory of differential equations" , Proc. 4-th All-Union Math. Congress (1961) , 2 , Leningrad (1964) pp. 432–437 (In Russian) |

[4] | N.N. [N.N. Bogolyubov] Bogoliubov, Yu.A. [Yu.A. Mitropol'skii] Mitropoliskii, A.M. [A.M. Samoilenko] Samolenko, "The method of accelerated convergence in non-linear mechanics" , Springer (1976) (Translated from Russian) |

[5] | Yu.A. Mitropol'skii, "Problems of the asymptotic theory of nonstationary vibrations" , D. Davey (1965) (Translated from Russian) |

[6] | Yu.A. Mitropol'skii, O.B. Lykova, "Integral manifolds in non-linear mechanics" , Moscow (1973) (In Russian) |

#### Comments

Nowadays integral manifolds are usually called invariant manifolds. Basic theorems on the permanence of invariant manifolds under perturbations are: 1) Fenichel's theorem, in case the Lyapunov exponents (cf. Lyapunov characteristic exponent) in the directions transversal to the manifold are larger in absolute value than those in directions parallel to the manifold, cf. [a1]; and 2) the Kolmogorov–Arnol'd–Moser theorem on persistence of quasi-periodic solutions in perturbations of integrable Hamiltonian systems (cf. Integrable system; Hamiltonian system; [a2]).

#### References

[a1] | M.W. Hirsch, C. Pugh, M. Shub, "Invariant manifolds" , Springer (1977) |

[a2] | V.I. Arnol'd, "Mathematical methods of classical mechanics" , Springer (1978) pp. Appendix 8 (Translated from Russian) |

[a3] | J. Guckenheimer, P. Holmes, "Non-linear oscillations, dynamical systems, and bifurcations of vector fields" , Springer (1983) |

**How to Cite This Entry:**

Integral manifold.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Integral_manifold&oldid=17392