  We consider the Hardy-Littlewood-Sobolev inequality on mixed-norm Lebesgue
spaces. We give a complete characterization of indices $\vec p$ and $\vec q$
such that the Riesz potential is bounded from $L^{\vec p}$ to $L^{\vec q}$,
including all the endpoint cases. As a result, we get the mixed-norm
Hardy-Littlewood-Sobolev inequality.
