  We analyze general properties of the effective Nambu two-particle vertex and
its renormalization group flow in a spin-singlet superconductor. In a fully
spin-rotation invariant form the Nambu vertex can be expressed by only three
distinct components. Solving exactly the flow of a mean-field model with
reduced BCS and forward scattering interactions, we gain insight into the
singularities in the momentum and energy dependences of the vertex at and below
the critical energy scale for superconductivity. Using a decomposition of the
vertex in various interaction channels, we manage to isolate singular momentum
and energy dependences in only one momentum and energy variable for each term,
such that the singularities can be efficiently parametrized.
