## On a nonlinear degenerate parabolic equation in infiltration or evaporation through a porous medium.(English)Zbl 0634.35042

The paper under review addresses the equation
$u(x,t)_t=\Phi(u(x,t))_{xx} +b(u(x,t))_x$ where $$\Phi$$ and $$b$$ are real continuous functions, $$t>0$$, and $$x$$ varies (i) on the whole real line, (ii) on a (bounded) interval, (iii) on a half-line; in the last two cases, non- homogeneous Dirichlet boundary conditions are provided at the finite endpoint(s); in any case, the problem is supplemented with initial data. This equation occurs in modelling infiltration or evaporation in a porous medium: for this reason, $$\Phi(u)$$ is assumed to “degenerate”, i.e., to have a vanishing derivative at $$u=0$$. The author first provide sufficient conditions for the existence of limit solutions, namely objects obtained as pointwise limits of approximate classical solutions, namely solutions of nondegenerate problems in appropriate cut-off domains. Then they give further conditions in order such limit solution to be weak solutions. In this context, disproving a conjecture put forward in some previous literature on the subject, they establish the dependency of the modulus of continuity of the weak solution of the transport term $$b(u)_x$$. Finally, the author prove uniqueness results, providing at the same time a thorough and unified discussion of the result in this direction obtained so far.

### MSC:

 35K60 Nonlinear initial, boundary and initial-boundary value problems for linear parabolic equations 35A35 Theoretical approximation in context of PDEs 35D99 Generalized solutions to partial differential equations 35D30 Weak solutions to PDEs 35K20 Initial-boundary value problems for second-order parabolic equations 35K65 Degenerate parabolic equations
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