Sobolev embedding for $M^{1,p}$ spaces is equivalent to a lower bound of the measure

Abstract

It has been known since 1996 that a lower bound for the measure, $\mu (B(x,r))\ge b{r}^s$, implies Sobolev embedding theorems for Sobolev spaces $M^{1,p}$ defined on metric-measure spaces. We prove that, in fact Sobolev embeddings for $M^{1,p}$ spaces are equivalent to the lower bound of the measure.