  Let $D$ be an invertible multiplication operator on $L^2(X, \mu)$, and let
$A$ be a bounded operator on $L^2(X, \mu)$. In this note we prove that $\|A\|^2
\le \|D A\| \, \|D^{-1} A\|$, where $\|\cdot\|$ denotes the operator norm. If,
in addition, the operators $A$ and $D$ are positive, we also have $w(A)^2 \le
w(D A) \, w(D^{-1} A)$, where $w$ denotes the numerical radius.
