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Regularity of the free boundary in parabolic phase-transition problems. (English) Zbl 0891.35164
The comprehensive paper is devoted to the study of regularity properties of the free boundary for general parabolic two-phase free boundary problems in several space variables. An important application of this type of phase-transition problems is the two-phase Stefan problem describing the melting/solidification of a material with a solid-liquid interphase.
The authors consider (so-called) viscosity solutions whose free boundary is given (locally) by a Lipschitz graph. In general, one cannot expect the smoothing of the free boundary $$F$$. This is shown by means of a counter example for the Stefan problem at the end of the paper. Therefore, a class of (in a sense non-degenerate) problems is studied, for which the authors prove that the regularity of the free boundary can be pushed to $$C^1$$ and the assumed viscosity solution is also a classical solution. The non-degeneracy condition states, roughly speaking, that the two heat fluxes are not vanishing simultaneously on $$F$$.

##### MSC:
 35R35 Free boundary problems for PDEs 80A22 Stefan problems, phase changes, etc. 35K55 Nonlinear parabolic equations
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##### References:
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