Title:

Inertial manifolds for semilinear parabolic equations which do not satisfy the spectral gap condition

An inertial manifold (IM) is one of the key objects in the modern theory of dissipative systems generated by partial differential equations (PDEs) since it allows us to describe the limit dynamics of the considered system by the reduced finitedimensional system of ordinary differential equations (ODEs). It is well known that the existence of an IM is guaranteed when the so called spectral gap conditions are satisfied, whereas their violation leads to the possibility of an infinitedimensional limit dynamics, at least on the level of an abstract parabolic equation. However, these conditions restrict greatly the class of possible applications and are usually satisfied in the case of one spatial dimension only. Despite many efforts in this direction, the IMs in the case when the spectral gap conditions are violated remain a mystery especially in the case of parabolic PDEs. On the one hand, there is a number of interesting classes of such equations where the existence of IMs is established without the validity of the spectral gap conditions and, on the other hand there were no examples of dissipative parabolic PDEs where the nonexistence of an IM is rigorously proved. The main aim of this thesis is to bring some light on this mystery by the comprehensive study of three model examples of parabolic PDEs where the spectral gap conditions are not satisfied, namely, 1D reactiondiffusionadvection (RDA) systems (see Chapter 3), the 3D CahnHilliard equation on a torus (see Chapter 4) and the modified 3D NavierStokes equations (see Chapter 5). For all these examples the existence or nonexistence of IM was an open problem. As shown in Chapter 3, the existence or nonexistence of an IM for RDA systems strongly depends on the boundary conditions. In the case of Dirichlet or Neumann boundary conditions, we have proved the existence of an IM using a specially designed nonlocal in space diffeomorphism which transforms the equations to the new ones for which the spectral gap conditions are satisfied. In contrast to this, in the case of periodic boundary conditions, we construct a natural example of a RDA system which does not possess an IM. In Chapters 4 and 5 we develop an extension of the socalled spatial averaging principle (SAP) (which has been suggested by Sell and MalletParet in order to treat scalar reactiondiffusion equation on a 3D torus) to the case of 4th order equations where the nonlinearity loses smoothness (the CahnHilliard equation) as well as for systems of equations (modified NavierStokes equations).
