The Bolzano mean-value theorem and partial differential equations

We study the existence of solutions to abstract equations of the form 0 = Au+F(u), u is an element of K subset of 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 a perturbation F satisfies a certain tangency condition. Such problems are closely related to the so-called Poincare Miranda theorem, being the multidimensional counterpart of the celebrated Bolzano intermediate value theorem. In fact our main results should be regarded as infinite-dimensional variants of Bolzano and Miranda Poincare 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. (C) 2017 Elsevier Inc. All rights reserved.