Traces of Sobolev functions on the Ahlfors sets of Carnot groups

We prove the converse of the trace theorem for the functions of the Sobolev spaces $W^l_p$ on a Carnot group on the regular closed subsets called Ahlfors $d$-sets (the direct trace theorem was obtained in one of our previous publications). The theorem generalizes Johnsson and Wallin's results for Sobolev functions on the Euclidean space. As a consequence we give a theorem on the boundary values of Sobolev functions on a domain with smooth boundary in a two-step Carnot group. We consider an example of application of the theorems to solvability of the boundary value problem for one partial differential equation.