We study the existence of solutions to abstract equations of the form 0 = Au + F (u), u ∈ K ⊂ E, where A is an abstract differential operator acting in a Banach space E, K is a closed convex set of constraints being invariant with respect to resolvents of A and perturbations are subject to different tangency condition. Such problems are closely related to the so-called Poicaré-Miranda theorem, being the multi-dimensional counterpart of the celebrated Bolzano intermediate value theorem. In fact our main results can and should be regarded as infinite-dimensional variants of Bolzano and Miranda-Poincaré theorems. Along with single-valued problems we deal with set-valued ones, yielding the existence of the so-called constrained equilibria of set-valued maps. The abstract results are applied to show existence of (strong) steady state solutions to some weakly coupled systems of drift reaction-diffusion equations or differential inclusions of this type. In particular we get the existence of strong solutions to the Dirichlet, Neumann and periodic boundary problems for elliptic partial differential inclusions under the presence of state constraints of different type. Certain aspects of the Bernstein theory for bvp for second order ODE are studied, too. No assumptions concerning structural coupling (monotonicity, cooperativity) are undertaken.

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We prove a Brezis-Kato-type regularity result for weak solutions to the biharmonic nonlinear equationThe regularity results give rise to the existence of ground state solutions provided that g has a general subcritical growth at infinity. We also conceive a new biharmonic logarithmic Sobolev inequalityfor a constant 0 < C < 2 πeN 2 and we characterize its minimizers.

Sometimes it is not possible to prove the uniqueness of the weak solutions for problems of mathematical physics, but it is possible to bootstrap their regularity to the regularity of strong solutions which are unique. In this paper we formulate an abstract setting for such class of problems and we provide the conditions under which the global attractors for both strong and weak solutions coincide and the fractal dimension of the common attractor is finite. We present two problems belonging to this class: planar Rayleigh-Bénard flow of thermomicropolar fluid and surface quasigeostrophic equation on torus.