The Sobolev Embeddings Are Usually Sharp

Let s ∈ R and p ≥ 1; the Sobolev space Lp,s(Rd) is the space of tempered distributions f such that (Id−∆)s/2 f ∈ Lp, where (Id−∆)s/2 is the Fourier multiplier by (1 + |ξ|2)s/2. If s > d/p, then Lp,s is composed of continuous functions; more precisely, the Sobolev embeddings state that Lp,s↩Cs−d/p, see [24, Chapter 11]. In order to state in which sense this embedding is sharp, we need to recall the notion of pointwise Hölder exponent.