  In 1990 F. Chiarenza and M. Frasca published a paper in which they
generalized a result of C. Fefferman on estimates of the integral of $|bu|^{p}$
through the integral of $|Du|^{p}$ for $p>1$. Formally their proof is valid
only for $d\geq 3$. We present here further generalization with a different
proof in which $D $ is replaced with the fractional power of the Laplacian for
any dimension $d\geq 1$.
