Relaxation of the incompressible porous media equation

It was shown recently by Cordoba, Faraco and Gancedo that the 2D porous media equation admits weak solutions with compact support in time. The proof, based on the convex integration framework, uses ideas from the theory of laminates, in particular T4 configurations. In this note we calculate the explicit relaxation of IPM, thus avoiding T4 configurations. We then use this to construct weak solutions to the unstable interface problem (the Muskat problem), as a byproduct shedding new light on the gradient flow approach introduced by Otto.Comment: 19 pages, no figures, some misprints correcte

The failure of spectral synthesis on some types of discrete Abelian groups

AbstractThe problem of spectral synthesis on arbitrary Abelian groups is solved in the negative

Spectral Synthesis on Varieties

In his classical paper, L. Schwartz proved that on the real line, in every linear translation invariant space of continuous complex valued functions, which is closed under compact convergence the exponential monomials span a dense subspace. He studied so-called local ideals in the space of Fourier transforms, and his proof based on the observation that, on the one hand, these local ideals are completely determined by the exponential monomials in the space, and, on the other hand, these local ideals completely determine the space itself. D.I.Gurevich used a similar idea of localization to give counterexamples for Schwartz's theorem in higher dimension. This localization process depends on differential operators and differentiability properties of the Fourier transforms. We note that in his two papers R.J. Elliott used somewhat similar ideas, but unfortunately, some of his proofs and results are incorrect. In this paper we show that the ideas of localization can be extended to general locally compact Abelian groups using abstract derivations on the Fourier algebra of compactly supported measures. Based this method we present necessary and sufficient conditions for spectral synthesis for varieties on locally compact Abelian groups