  The condition mentioned in the title is equivalent to the representability of
$f$ as the quotient $f=v_1/v_2$, where $v_1$ and $v_2$ obey the inequalities
$|R_j v_i| \leq C |v_i|$ for $i=1,2$ and $j=1,\ldots, n$. Here, $R_1,\ldots,
R_n$ are the Riesz transformations.
