 Solution

Let $c$ be the center and $\ell$ the side length of $Q$. The zero integral of $f$ lets us subtract a constant kernel value:Choose $L=4\sqrt{d}$. For $x$ outside the concentric cube $\hat{Q}$ of side length $L\ell$, and for $y\in Q$,Thus the domain of integration in $x$ is contained in $\{|x-c|>2|y-c|\}$. After the translations $X=x-c$ and $Y=y-c$, the Hörmander integral kernel condition givesIf $Y=0$ the difference is zero and the same inequality holds. Apply the triangle inequality and Tonelli theorem to obtain the cancellation estimate outside a dilated cube:In particular the constant on the right is the same $C$ as in the hypothesis; only the dilation $L$ depends on the dimension.