An improved dense class in Sobolev spaces to manifolds

Abstract

We consider the strong density problem in the Sobolev space $ W^{s,p}(Q^{m};\mathscr{N}) $ of maps with values into a compact Riemannian manifold $ \mathscr{N} $. It is known, from the seminal work of Bethuel, that such maps may always be strongly approximated by $ \mathscr{N} $-valued maps that are smooth outside of a finite union of $ (m -\lfloor sp \rfloor - 1) $-planes. Our main result establishes the strong density in $ W^{s,p}(Q^{m};\mathscr{N}) $ of an improved version of the class introduced by Bethuel, where the maps have a singular set without crossings. This answers a question raised by Brezis and Mironescu. In the special case where $ \mathscr{N} $ has a sufficiently simple topology and for some values of $ s $ and $ p $, this result was known to follow from the method of projection, which takes its roots in the work of Federer and Fleming. As a first result, we implement this method in the full range of $ s $ and $ p $ in which it was expected to be applicable. In the case of a general target manifold, we devise a topological argument that allows to remove the self-intersections in the singular set of the maps obtained via Bethuel's technique.