21 oct 2020 -- 18:00 [open in google calendar]

Colloquium Dipartimento di Matematica di Pisa - online

In order to join the seminar, please fill in the mandatory participation form before October 20th. Further information and instructions will be sent afterwards to the online audience. The streaming is also available on the Youtube channel https://youtu.be/D-GUe6MCwZc

**Abstract.**

We consider an equation in divergence form with a singular-degenerate weight \[
-\mathrm{div}(

y

^a A(x,y)\nabla u)=

y

^a f(x,y,u)\; \quad\textrm{or}\; \textrm{div}(

y

^aF(x,y,u))\;,
\]
We first study the regularity of the nodal sets of solutions in the linear case. Next, when the r.h.s. does not depend on $u$, under suitable regularity assumptions for the matrix $A$ and $f$ (resp. $F$) we prove H\"older continuity of solutions and possibly of their derivatives up to order two or more (Schauder estimates). In addition, we show stability of the $C^{0,\alpha}$ and $C^{1,\alpha}$ a priori bounds for approximating problems in the form
\[
-\mathrm{div}((\varepsilon^2+y^2)^a A(x,y)\nabla u)=(\varepsilon^2+y^2)^a f(x,y)\; \quad\textrm{or}\; \textrm{div}((\varepsilon^2+y^2)^aF(x,y))
\]
as $\varepsilon\to 0$. Finally, we derive $C^{0,\alpha}$ and $C^{1,\alpha}$ bounds for inhomogenous Neumann boundary problems as well. Our method is based upon blow-up and appropriate Liouville type theorems.