Very weak solutions to the boundary-value problem of the homogenous heat equation

Abstract

We consider the homogeneous heat equation in a domain $Ω$ in $\mathbb{R}^n$ with vanishing initial data and the Dirichlet boundary condition. We are looking for solutions in $W^{r,s}_{p,q}(Ω\times(0,T))$, where $r < 2$, $s < 1$, $1 \leq p < \infty$, $1 \leq q \leq \infty$. Since we work in the $L_{p,q}$ framework any extension of the boundary data and integration by parts are not possible. Therefore, the solution is represented in integral form and is referred as \emph{very weak} solution. The key estimates are performed in the half-space and are restricted to $L_q(0,T;W^α_p(Ω))$, $0 \leq α< \frac{1}{p}$ and $L_q(0,T;W^α_p(Ω))$, $α\leq 1$. Existence and estimates in the bounded domain $Ω$ follow from a perturbation and a fixed point arguments.