  The functional renormalisation group is applied to the effective action for
scattering of two nonrelativistic fermions. The resulting physical effective
action is shown to contain the correct threshold singularity. The corresponding
"bare" action respects Galilean invariance up to second order in momenta.
Beyond that order it contains terms that violate this symmetry and, for the
particular regulator considered, nonanalytic third-order terms. The
corresponding potential can be expanded around a nontrivial fixed point using
the power counting appropriate to a system with large scattering length.
