On Luzin N-property and uncertainty principle for the Sobolev mappings

We study Luzin N-property with respect to the Hausdorff measures for Sobolev spaces W^k_p(R^n,R^d). We prove that such N-property holds except for one critical dimensional value t_*=n-(k-1)p; for this critical value the N-property fails in general, and we constructed the corresponding nontrivial counterexample (based on the theory of lacunary Fourier series). Nevertheless, this N-property holds if we assume in addition that the highest k-derivatives belongs to the Lorentz space L_{p,1} instead of L_p. We extend these results to the case of fractional Sobolev spaces as well. Also, we establish some Fubini type theorems for $N$-properties and discuss their applications to the Morse--Sard theorem and its recent extensions.