# Integral manifold

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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)