Abstract
Let L be a linear elliptic operator in two dimensions with analytic coefficients and of second order, andu(x, y) a solution of Lu=0 in a simply connected domain ω with rectifiable boundary Γ. Suppose ψ(x, y) analytic on ω∪Γ and L ψ≠0 there.H is shown that ifu and ψ coincide with first derivatives on an open portion Γ0 of Γ, then Γ0 permits the representation λ=x (θ),y=y (θ) withx(θ),y(θ)analytic functions of a real parameter θ.
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References
Lewy H., Stampacchia G.,On the regularity of the solutions of a Variational inequality. Comm. Pure Appl. Math.22 (1969), 133–188.
Lewy H.,On the reflection laws of second order differential equations in two independent variables, Bull. Amer. Math. Soc.65 (1959).
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Kinderlehrer D., Nirenberg L.,Regularity in free boundary problems, Ann. Scuola Norm. Sup. Ser. IV4 (1977) treats similar problems inn≧2 dimensions, assuming Hölder continuity of the tangent manifold.
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Lewy, H., Zhiyuan, T. On free boundary problems in two dimensions. Rend. Circ. Mat. Palermo 34, 325–336 (1985). https://doi.org/10.1007/BF02844529
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DOI: https://doi.org/10.1007/BF02844529