A control on the set where a Green's function vanishes
We prove that every homogeneous Carnot group can be lifted to a free homogeneous Carnot group. Though following the ideas of Rothschild and Stein, we give simple and self-contained arguments, providing a constructive proof, as shown in the examples.
The two-phase free boundary value problem for the Navier-Stokes system is considered in a situation where the initial interface is close to a halfplane. We extract the boundary symbol which is crucial for the dynamics of the free boundary and present an analysis of this symbol. Of particular interest are its singularities and zeros which lead to refined mapping properties of the corresponding operator.
If is a polynomial in such that integrable, then the inverse Fourier transform of is a fundamental solution to the differential operator . The purpose of the article is to study the dependence of this fundamental solution on the polynomial . For it is shown that can be analytically continued to a Riemann space over the set of all polynomials of the same degree as . The singularities of this extension are studied.