It is shown that, under a Calderón type condition on the function ϕ, the continuous open mappings that belong to the Orlicz-Sobolev classes W 1,ϕ loc have total differential almost everywhere; this generalizes the well-known theorems of Gehring-Lehto-Menchoff in the case of R 2 and of Väisälä in R n , n ≥ 3. Appropriate examples show that the Calderón type condition is not only sufficient but also necessary. Moreover, under the same condition on ϕ, it is also proved that the continuous mappings of class W 1,ϕ loc and, in particular, of class W 1,p loc for p > n−1 have Lusin's (N )-property on a.e. hyperplane. On that basis, it is shown that, under the same condition on ϕ, the homeomorphisms f with finite distortion of class W 1,ϕ loc and, in particular, those belonging to W 1,p loc for p > n−1, are what is called lower Q-homeomorphisms, where Q is equal to their outer dilatation K f ; also, they are so-called ring Q * -homeomorphisms with Q * = K n−1 f . The latter fact makes it possible to fully apply the theory of the boundary and local behavior of the ring and lower Q-homeomorphisms, as developed earlier by the authors, to the study of mappings in the Orlicz-Sobolev classes.
Part 1. Differentiability and behavior of Hausdorff 's measures in the Orlicz-Sobolev classes2010 Mathematics Subject Classification. Primary 46E35. Key words and phrases. Moduli of families of curves and surfaces, mappings with bounded and finite distortion, differentiability, Lusin and Sard properties, Sobolev classes, Orlicz-Sobolev classes, boundary and local behavior.