We study the local behavior of bounded local weak solutions to a class of anisotropic singular equations that involves both non-degenerate and singular operators. Throughout a parabolic approach to expansion of positivity we obtain the interior Holde
r continuity, and some integral and pointwise Harnack inequalities.
We introduce Fundamental solutions of Barenblatt type for the equation $u_t=sum_{i=1}^N bigg( |u_{x_i}|^{p_i-2}u_{x_i} bigg)_{x_i}$, $p_i >2 quad forall i=1,..,N$, on $Sigma_T=mathbb{R}^N times[0,T]$, and we prove their importance for the regularity properties of the solutions.
We give a proof of Holder continuity for bounded local weak solutions to the equation $u_t= sum_{i=1}^N (|u_{x_i}|^{p_i-2} u_{x_i})_{x_i}$, in $Omega times [0,T]$, with $Omega subset subset mathbb{R}^N$, under the condition $ 2<p_i<bar{p}(1+2/N)$ f
or each $i=1,..,N$, being $bar{p}$ the harmonic mean of the $p_i$s, via recently discovered intrinsic Harnack estimates. Moreover we establish equivalent forms of these Harnack estimates within the proper intrinsic geometry.
We prove a Harnack inequality for positive solutions of a parabolic equation with slow anisotropic spatial diffusion. After identifying its natural scalings, we reduce the problem to a Fokker-Planck equation and construct a self-similar Barenblatt so
lution. We exploit translation invariance to obtain positivity near the origin via a self-iteration method and deduce a sharp anisotropic expansion of positivity. This eventually yields a scale invariant Harnack inequality in an anisotropic geometry dictated by the speed of the diffusion coefficients. As a corollary, we infer Holder continuity, an elliptic Harnack inequality and a Liouville theorem.
We shall establish the interior Holder continuity for locally bounded weak solutions to a class of parabolic singular equations whose prototypes are begin{equation} u_t= abla cdot bigg( | abla u|^{p-2} abla u bigg), quad text{ for } quad 1<p<2, end
{equation} and begin{equation} u_{t}- abla cdot ( u^{m-1} | abla u |^{p-2} abla u ) =0 , quad text{for} quad m+p>3-frac{p}{N}, end{equation} via a new and simplified proof using recent techniques on expansion of positivity and $L^{1}$-Harnack estimates.
In recent years, many papers have been devoted to the regularity of doubly nonlinear singular evolution equations. Many of the proofs are unnecessarily complicated, rely on superfluous assumptions or follow an inappropriate approximation procedure. T
his makes the theory unclear and quite chaotic to a nonspecialist. The aim of this paper is to fix all the misprints, to follow correct procedures, to exhibit, possibly, the shortest and most elegant proofs and to give a complete and self-contained overview of the theory.
