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I shall discuss joint work with John L. Lewis on the solvability of boundary value problems for the heat equation in non-cylindrical (i.e., time-varying) domains, whose boundaries are in some sense minimally smooth in both space and time. The emphasis will be on the Neumann problem with data in $L^p$. A somewhat surprising feature of our results is that, in contrast to the cylindrical case, the optimal results hold when $p=2$, with the situation getting progressively worse as $p$ approaches $1$. In particular,...

If $B_{\alpha ,p}$ denotes the Bessel capacity of subsets of Euclidean $n$-space, $\alpha \>0$, $1\<p\<\infty$, naturally associated with the space of Bessel potentials of $L^p$-functions, then our principal result is the estimate: for $1\<\alpha p\le n$, there is a constant $C=C(\alpha ,p,n)$ such that for any set $E$ $$min\{B_{\alpha ,p}(E\cap Q),B_{\alpha ,p}(E^c\cap Q\left)\right\}\le C·B_{\alpha ,p}(Q\cap {\partial }_{f}E)$$ for all open cubes $Q$ in $n$-space. Here ${\partial }_{f}E$ is the boundary of the $E$ in the $(\alpha ,p)$-fine topology i.e. the smallest topology on $c$-space that makes the associated $(\alpha ,p)$-linear potentials continuous there. As a consequence, we deduce that...

We construct bounded domains D not equal to a ball in n ≥ 3 dimensional Euclidean space, R, for which ∂D is homeomorphic to a sphere under a quasiconformal mapping of R and such that n - 1 dimensional Hausdorff measure equals harmonic measure on ∂D.

We establish L and L bounds for a class of square functions which arises in the study of singular integrals and boundary value problems in non-smooth domains. As an application, we present a simplified treatment of a class of parabolic smoothing operators which includes the caloric single layer potential on the boundary of certain minimally smooth, non-cylindrical domains.

Let $\Omega$ be a bounded simply connected domain in the complex plane, $ℂ$. Let $N$ be a neighborhood of $\partial \Omega$, let $p$ be fixed, $1\<p\<\infty ,$ and let $\widehat{u}$ be a positive weak solution to the $p$ Laplace equation in $\Omega \cap N.$ Assume that $\widehat{u}$ has zero boundary values on $\partial \Omega$ in the Sobolev sense and extend $\widehat{u}$ to $N\setminus \Omega$ by putting $\widehat{u}\equiv 0$ on $N\setminus \Omega .$ Then there exists a positive finite Borel measure $\widehat{\mu }$ on $ℂ$ with support contained in $\partial \Omega$ and such that $$\begin{array}{c}\hfill \int |\nabla \widehat{u}{|}^{p-2}\langle \nabla \widehat{u},\nabla \phi \rangle dA=-\int \phi d\widehat{\mu }\end{array}$$ whenever $\phi \in C_0^{\infty }\left(N\right).$ If $p=2$ and if $\widehat{u}$ is the Green function for $\Omega$ with pole at $x\in \Omega \setminus \overline{N}$ then the measure $\widehat{\mu }$ coincides...

Let $\Omega \subset {ℝ}^n,n\ge 3$, and let $p,1<p<\infty ,p\ne 2$, be given. In this paper we study the dimension of $p$-harmonic measures that arise from non-negative solutions to the $p$-Laplace equation, vanishing on a portion of $\partial \Omega$, in the setting of $\delta$-Reifenberg flat domains. We prove, for $p\ge n$, that there exists $\tilde{\delta }=\tilde{\delta }(p,n)>0$ small such that if $\Omega$ is a $\delta$-Reifenberg flat domain with $\delta <\tilde{\delta }$, then $p$-harmonic measure is concentrated on a set of $\sigma$-finite $H^{n-1}$-measure. We prove, for $p\ge n$, that for sufficiently flat Wolff snowflakes the Hausdorff dimension of $p$-harmonic measure...