$\psi$. For the truncation of the effective action we use a vertex expansion. Due to the nonrelativistic dispersion relation and the instantaneous interaction, the propagator of the field $\psi$ and the Yukawa-like coupling $\sim \Phi^* \psi_1\psi_2+c.c.$ are independent of the scale $k$. Also, no additional interaction vertex involving the field $\psi$ is generated by the renormalization group flow. Indeed, all one-loop Feynman diagrams that contribute to the flow equations of the above quantities consist of a closed tour of particles and vanish in the nonrelativistic few-body limit. This feature is a manifestation of the fact that the nonrelativistic few-body problems decouple in the sence that for example the three-body problem can be solved independent of the four-body problem. More general, the flow equations for correlation functions that determine the $n$-body problem are independent of the correlation functions determining the $n+1$-body problem. For a more detailed discussion of these matters we refer to ref. [@SFdiss]. As another consequence of the decoupling feature, the flow equation for the propagator of the field $\Phi$ depends only on the propagator of the field $\psi$ and the Yukawa-like vertex but is independent of the higher vertex functions. This implies that the flow equation for the 