On the global existence for the Muskat problem

The Muskat problem models the dynamics of the interface between two incompressible immiscible fluids with different constant densities. In this work we prove three results. First we prove an L-2(R) maximum principle, in the form of a new "log" conservation law (3) which is satisfied by the equation (1) for the interface. Our second result is a proof of global existence for unique strong solutions if the initial data is smaller than an explicitly computable constant, for instance parallel to f parallel to(1) <= 1/5. Previous results of this sort used a small constant epsilon << 1 which was not explicit [7, 19, 9, 14]. Lastly, we prove a global existence result for Lipschitz continuous solutions with initial data that satisfy parallel to f(0)parallel to(L infinity) < infinity and parallel to partial derivative(x)f(0)parallel to(L infinity) < 1. We take advantage of the fact that the bound parallel to partial derivative(x)f(0)parallel to(L infinity) < 1 is propagated by solutions, which grants strong compactness properties in comparison to the log conservation law.