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A minimization problem and the regularity of solutions in the presence of a free boundary. (English) Zbl 0545.35013
The author deals with the minimization problem $J(u)=\int_{G}(| \nabla u|^ 2/2+u^{\gamma})dx=\min_{w\in K}J(w),\quad u\in K,$ where G is an open bounded set in $$R^ n$$ with Lipschitz boundary $$\partial G$$, $$0<\gamma<1$$ a fixed constant and $$K=\{w\in H^ 1(G):w\geq 0,$$ $$w=u_ 0|_{\partial G}\}$$ with $$u_ 0\in H^ 1(G)$$ given, $$u_ 0\geq 0$$. It is shown that at least one minimizer $$u\in K$$ exists. Also u is shown to be subharmonic in G. The main result of the paper is the interior regularity of u, i.e. for any compact subset $$H\subset G$$, $$u\in C^{1,\beta -1}(H)$$ with $$\beta =2/(2-\gamma).$$
An interesting ingredient in the proof is the observation that the functional J preserves minimizers under a particular scaling. In the extreme cases $$\gamma =1$$ and $$\gamma =0$$ the same problem was previously studied by L. A. Caffarelli [Commun. Partial Differ. Equations 5, 427-448 (1980; Zbl 0437.35070)] and by H. Alt and L. A. Caffarelli [J. Reine Angew. Math. 325, 105-144 (1981; Zbl 0449.35105)].
Reviewer: V.Mustonen

MSC:
 35B65 Smoothness and regularity of solutions to PDEs 35A15 Variational methods applied to PDEs 35R35 Free boundary problems for PDEs 49J20 Existence theories for optimal control problems involving partial differential equations
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