Embeddings of homogeneous Sobolev spaces on the entire space

We completely characterize the validity of the inequality parallel to u parallel to(Y(Rn)) <= C parallel to del(m)u parallel to(X(Rn)), where X and Y are rearrangement-invariant spaces, by reducing it to a considerably simpler one-dimensional inequality. Furthermore, we fully describe the optimal rearrangement-invariant space on either side of the inequality when the space on the other side is fixed. We also solve the same problem within the environment in which the competing spaces are Orlicz spaces. A variety of examples involving customary function spaces suitable for applications is also provided.