In this paper we consider the so-called double-phase problem where the phase transition takes place across
the zero level "surface" of the minimizer of the functional
$$
J_{p,q}(v,\Omega) = \int_\Omega \left( |\nabla v^+|^p + |\nabla v^-|^q \right) dx.
$$
We prove that the minimizer exists, and is H\"older regular. From here, using an intrinsic variation,
one can prove a weak formulation of the free boundary condition across the zero level surface, that formally
can be represented as
$$
(q-1)|\nabla u^-|^q = (p-1) |\nabla u^+|^p, \quad \hbox{on } \partial \{u > 0\}.
$$
We prove that the free boundary is $C^{1,\alpha}$ a.e.
with respect to the measure $\Delta_p u^+$, whose support is of $\sigma$-finite $(n-1)$-dimensional Hausdorff measure.

ArticleRadial symmetry for an elliptic PDE with a free boundary

In this paper we prove symmetry for solutions to the semi-linear elliptic equation
$$
\Delta u = f(u) \quad \hbox{ in } B_1,
\qquad 0 \leq u < M, \quad \hbox{ in } B_1, \qquad u = M, \quad \hbox{ on } \partial B_1,
$$
where $M>0$ is a constant, and $B_1$ is the unit ball.
Under certain assumptions on the r.h.s. $f (u)$, the $C^1$-regularity of the free boundary $\partial \{u>0\}$
and a second order asymptotic expansion for $u$ at free boundary points,
we derive the spherical symmetry of solutions. A key tool, in addition to the classical moving plane technique, is a
boundary Harnack principle (with r.h.s.) that replaces Serrin's celebrated boundary point lemma, which is not available in our case due to lack of $C^2$-regularity of solutions.

ArticleRegularity of the free boundary for a parabolic cooperative system

In this paper we study the following parabolic system
\begin{equation*}
\Delta {\bf u} -\partial_t {\bf u} =|{\bf u}|^{q-1}{\bf u}\,\chi_{\{ |{\bf u}|>0 \}}, \qquad {\bf u} = (u^1, \cdots , u^m) \ ,
\end{equation*}
with free boundary $\partial \{|{\bf u} | >0\}$.
For $0\leq q< 1$, we prove optimal growth rate for solutions ${\bf u} $ to the above system near free boundary points, and show that in a uniform neighbourhood of any a priori well-behaved free boundary point the free boundary is $C^{1, \alpha}$ in space directions and half-Lipschitz
in the time direction.

ArticleThe Inhomogeneous Boundary Harnack Principle for Fully Nonlinear and p-Laplace equations

We prove a boundary Harnack principle in Lipschitz domains with small constant for fully nonlinear and $p$-Laplace type equations with a right hand side, as well as for the Laplace equation on nontangentially accessible domains under extra conditions. The approach is completely new and gives a systematic approach for proving similar results for a variety of equations and geometries.

ArticleLipschitz Regularity in Vectorial Linear Transmission Problems

We consider vector-valued solutions to a linear transmission problem, and we
prove that Lipschitz-regularity on one phase is transmitted to the next phase. More exactly, given a solution $u:B_1\subset {\mathbb R}^n \to {\mathbb R}^m$ to the elliptic system
\begin{equation*}
\hbox{div} ((A + (B-A)\chi_D )\nabla u) = 0 \quad \text{in }B_1,
\end{equation*}
where $A$ and $B$ are Dini continuous, uniformly elliptic matrices, we prove that if $\nabla u \in L^{\infty} (D)$ then $u$ is Lipschitz in $B_{1/2}$.
A similar result is also derived for the parabolic counterpart of this problem.

ArticleMonotonicity formulas for coupled elliptic gradient systems with applications

If you plan to formulate more than one problem all sharing the same background (e.g. they are all from the same paper) then please choose "Group", otherwise select "Single" option.

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