On the Morse-Sard property and level sets of $W^{n,1}$ Sobolev functions on ${\mathbb{R}}^n$

We establish Luzin N and Morse–Sard properties for functions from the Sobolev space $W^{n,1}({\mathbb{R}}^n)$. Using these results we prove that almost all level sets are finite disjoint unions of $C^1$–smooth compact manifolds of dimension $n-1$. These results remain valid also within the larger space of functions of bounded variation $B{V}_n({\mathbb{R}}^n)$. For the proofs we establish and use some new results on Luzin–type approximation of Sobolev and BV–functions by $C^k$–functions, where the exceptional sets have small Hausdorff content.