Sobolev homeomorphic extensions from two to three dimensions

We study the basic question of characterizing which boundary homeomorphisms of the unit sphere can be extended to a Sobolev homeomorphism of the interior in 3D space. While the planar variants of this problem are well-understood, completely new and direct ways of constructing an extension are required in 3D. We prove, among other things, that a Sobolev homeomorphism $\varphi \colon \mathbb R^2 \to \mathbb R^2$ in $W_{loc}^{1,p} (\mathbb R^2 , \mathbb R^2)$ for some $p\in [1,\infty )$ admits a homeomorphic extension $h \colon \mathbb R^3 \to \mathbb R^3$ in $W_{loc}^{1,q} (\mathbb R^3, \mathbb R^3)$ for $1\le q < \frac{3}{2}p$. Such an extension result is nearly sharp, as the bound $q=\frac{3}{2}p$ cannot be improved due to the H\"older embedding. The case $q=3$ gains an additional interest as it also provides an $L^1$-variant of the celebrated Beurling-Ahlfors extension result.