On Ribet's level-raising theorem four U(3)

A well-known theorem of Ken Ribet asserts that, under certain assumptions, a modular form (mod l) on Gamma (0)(N) can be "lifted" to yield a newform o n Gamma (0)(Nl) with the same modular Galois representation. For further pr ogress in the modular theory of automorphic forms one will need to understa nd this phenomenon for automorphic forms on reductive groups other than GL( 2). In this paper we prove such a result for the unitary group of rank 3, u nder suitable assumptions. The proof relies on the modular representation t heory of p-adic reductive groups.