Abstract. For 3D reaction-diffusion equations, we study the problem of existence or nonexistence of an inertial manifold that is normally hyperbolic or absolutely normally hyperbolic. We present a system of two coupled equations with a cubic nonlinearity which does not admit a normally hyperbolic inertial manifold. An example separating the classes of such equations admitting an inertial manifold and a normally hyperbolic inertial manifold is constructed. Similar questions concerning absolutely normally hyperbolic inertial manifolds are discussed. Keywords: reaction-diffusion equations, inertial manifold, normal hyperbolicity.
IntroductionThe existence of a smooth inertial manifold M for the dissipative parabolic equation in the infinite-dimensional Hilbert space implies [14,18,19] that its final dynamics (as t → +∞) is controlled by finitely many parameters. The additional property of normal hyperbolicity of the inertial manifold M guarantees the structural stability of this manifold. The stronger property of absolute normal hyperbolicity means one and the same hyperbolicity parameters for the entire M. So far, the existence of an inertial C 1 -manifold has been established for a rather narrow class of semilinear parabolic equations, while known examples of its nonexistence [2,15,16] seem to be somewhat artificial and are not related to problems of mathematical physics.The present paper deals with necessary conditions for the existence of the abovementioned two types of inertial manifolds of scalar and vector reaction-diffusion equations. For the 3D chemical kinetics equations with a cubic nonlinearity, we strive for 1 constructing examples separating the classes of problems admitting an inertial manifold, a normally hyperbolic inertial manifold, and an absolutely normally hyperbolic inertialmanifold. An example separating the first two possibilities is obtained for two-component systems. Namely, in Proposition 3.5 we construct an (uncoupled) system of such equations that has an inertial manifold but does not admit a normally hyperbolic inertial manifold. In particular, this system provides an example of an inertial manifold that is not normally hyperbolic. On the other hand, we present a system of two coupled reaction-diffusion equations of this type that do not admit a normally hyperbolic inertial manifold in the natural state space (Proposition 3.4). An example of a scalar 3D equation with a cubic nonlinearity without an absolutely normally hyperbolic inertial manifold is constructed. Note that the order of the polynomial nonlinearity in the chemical kinetics equations corresponds to the reaction order, which usually does not exceed 3. We also discuss how close the well-known sufficient conditions (the spectral jump condition and the spatial averaging principle) for the existence of strongly and weakly normally hyperbolic inertial manifolds are to being necessary.The paper is organized as follows. Section 1 contains elementary information about abstract semilinear parabolic equations. The necessary and suffi...