Abstract. In this paper we study a one phase free boundary problem for the p(x)-Laplacian with non-zero right hand side. We prove that the free boundary of a weak solution is a C 1,α surface in a neighborhood of every "flat" free boundary point. We also obtain further regularity results on the free boundary, under further regularity assumptions on the data. We apply these results to limit functions of an inhomogeneous singular perturbation problem for the p(x)-Laplacian that we studied in [25].
IntroductionIn this paper we study the following inhomogeneous free boundary problem for the p(x)-Laplacian: u ≥ 0 andThe p(x)-Laplacian serves as a model for a stationary non-newtonian fluid with properties depending on the point in the region where it moves. For example, such a situation corresponds to an electrorheological fluid. These are fluids such that their properties depend on the magnitude of the electric field applied to it. In some cases, fluid and Maxwell's equations become uncoupled and a single equation for the p(x)-Laplacian appears (see [33]).The free boundary problem P (f, p, λ * ) appears, for instance, in the limit of a singular perturbation problem that may model high activation energy deflagration flames in a fluid with electromagnetic sensitivity (see [25]). When p(x) ≡ 2 (in which case the p(x)-Laplacian coincides with the Laplacian) this singular perturbation problem was introduced by Zeldovich and Frank-Kamenetski in order to model these kind of flames in [37]. In this latter case, the right hand side f may come from nonlocal effects as well as from external sources (see [23]).The free boundary problem considered in this paper also appears in an inhomogeneous minimization problem that we study in [26] where we prove that minimizers are weak solutions to P (f, p, λ * ).In the present article we prove that the free boundary ∂{u > 0} -with u a weak solution of P (f, p, λ * )-is a smooth hypersurface in a neighborhood of every "flat" free boundary point.Key words and phrases. Free boundary problem, variable exponent spaces, regularity of the free boundary, singular perturbation, inhomogeneous problem.