A complete answer to the strong density problem in Sobolev spaces with values into compact manifolds

Abstract: We consider the problem of strong density of smooth maps in the Sobolev space $ W^{s,p}(Q^{m};\mathcal{N}) $, where $ 0 < s < +\infty $, $ 1 \leq p < +\infty $, $ Q^{m} $ is the unit cube in $ \mathbb{R}^{m} $, and $ \mathcal{N} $ is a smooth compact connected Riemannian manifold without boundary. Our main result fully answers the strong density problem in the whole range $ 0 < s < +\infty $: the space $ \mathcal{C}^{\infty}(\overline{Q}^{m};\mathcal{N}) $ is dense in $ W^{s,p}(Q^{m};\mathcal{N}) $ if and only if $ \pi_{[sp]}(\mathcal{N}) = {0} $. This completes the results of Bethuel ($ s=1 $), Brezis and Mironescu ($ 0 < s < 1 $), and Bousquet, Ponce, and Van Schaftingen ($ s = 2 $, $ 3 $, ...). We also consider the case of more general domains $ \Omega $, in the setting studied by Hang and Lin when $ s = 1 $.