  For single-particle nonrelativistic quantum mechanics, a Gamow state is an
eigenfunction of the Hamiltonian with complex eigenvalue. Gamow states are not
normalizable; they depend on time via the usual multiplier exp(-iEt)
supplemented by a cutoff at an expanding wavefront. Gamow states have been used
to extend nuclear shell models; they are to metastable states as normalizable
eigenfunctions are to bound states. In this paper we generalize Gamow states to
decays with multiple outgoing particles. We derive the exact form of the
expanding wavefront, even for relativistic outgoing particles. In the
non-relativistic limit we derive the exact form of the multi-particle Gamow
state contribution to the system propagator (Green's function).
