13 research outputs found
Boundary weak Harnack estimates and regularity for elliptic PDE in divergence form
We obtain a global extension of the classical weak Harnack inequality which
extends and quantifies the Hopf-Oleinik boundary-point lemma, for uniformly
elliptic equations in divergence form. Among the consequences is a boundary
gradient estimate, due to Krylov and well-studied for non-divergence form
equations, but completely novel in the divergence framework. Another
consequence is a new more general version of the Hopf-Oleinik lemma.Comment: 19 pages; some misprints corrected, the proof of Theorem 1.2 was
clarified and simplifie
A note on boundary point principles for partial differential inequalities of elliptic type
In this note we consider boundary point principles for partial differential
inequalities of elliptic type. Firstly, we highlight the difference between
conditions required to establish classical strong maximum principles and
classical boundary point lemmas for second order linear elliptic partial
differential inequalities. We highlight this difference by introducing a
singular set in the domain where the coefficients of the partial differential
inequality need not be defined, and in a neighborhood of which, can blow-up.
Secondly, as a consequence, we establish a comparison-type boundary point lemma
for classical elliptic solutions to quasi-linear partial differential
inequalities. Thirdly, we consider tangency principles, for elliptic weak
solutions to quasi-linear divergence structure partial differential
inequalities. We highlight the necessity of certain hypotheses in the
aforementioned results via simple examples.Comment: 17 pages. No figure
Lyapunov functions, Identities and the Cauchy problem for the Hele-Shaw equation
This article is devoted to the study of the Hele-Shaw equation. We introduce
an approach inspired by the water-wave theory. Starting from a reduction to the
boundary, introducing the Dirichlet to Neumann operator and exploiting various
cancellations, we exhibit parabolic evolution equations for the horizontal and
vertical traces of the velocity on the free surface. This allows to
quasi-linearize the equations in a very simple way. By combining these exact
identities with convexity inequalities, we prove the existence of hidden
Lyapunov functions of different natures. We also deduce from these identities
and previous works on the water wave problem a simple proof of the
well-posedness of the Cauchy problem. The analysis contains two side results of
independent interest. Firstly, we give a principle to derive estimates for the
modulus of continuity of a PDE under general assumptions on the flow. Secondly
we prove a convexity inequality for the Dirichlet to Neumann operator.Comment: We have added some reference