“…This has been favoured by a myriad of investigations on nonlocal equations of elliptic and parabolic type, whose solutions naturally belong to the spaces W s,p (R n ). A touch of recent contributions in this connection is furnished by [7,8,11,12,13,14,15,16,17,18,20,21,22,23,24,25,26,27,28,29,41,44,45,51,52,53,58,59,61,62,63,64,65,68,70,71,77,76,78,83,86]. Comprehensive treatments of the theory of fractional Sobolev spaces, as special instances of the more general Besov spaces, can be found e.g.…”