Sobolev spaces and Lagrange interpolation

In this short paper the discussion of the pointwise characterization of functions $f$ in the Sobolev space $W^{m,p}({\mathbb{R}}^n)$ given in the recent paper (Bojarski) is supplemented in \SS1 by a direct, essentially geometric, proof of the novel inequality (for $m>1$), appearing in Bojarski apparently for the first time, and involving the use of the $m$-th difference of the function $f$. Moreover in \SS2 some additional comments to the text in Bojarski are given and a natural class of Sobolev spaces in domains $G$ in ${\mathbb{R}}^n$ is defined. \SS3 contains some final remarks.