On the limiting behaviour of some nonlocal seminorms: a new phenomenon

In this note we study the behaviour as s → 0+ of some semigroup based Besov seminorms associated with a non-symmetric and hypoelliptic diffusion with a drift. Our results generalise a previous one of Maz'ya and Shaposhnikova for the classical fractional Sobolev spaces W s;p, and they also underscore a new phenomenon caused by the presence of the drift.