We prove a generalization of Demailly-Ein-Lazarsfeld's subadditivity formula
and Mustata's summation formula for multiplier ideals to the case of singular
varieties, using characteristic $p$ methods. As an application of our formula,
we improve Hochster-Huneke's result on the growth of symbolic powers of ideals
in singular affine algebras.Comment: 16 pages, AMS-LaTeX; v.2: Some minor change