Off-shell unitarity constraints and theK-matrix formalism

The recent results of Kouri and Levin on the properties of the single-channel two-particle off-shell transition amplitude which follow from off-shell unitarity are shown to be readily obtainable from results in the published literature. It is also pointed out that the use of the damping (or Heitler) equation as a realization of off-shell unitarity leads to no new information except on the off-shell structure of the $K$-matrix amplitude. Finally, the implications of off-shell unitarity in some (two-particle) multichannel situations are deduced, thereby extending the results of previous investigations.