  Nonrelativistic two-body scattering by a short-ranged potential is studied
using the renormalisation group. Two fixed points are identified: a trivial one
and one describing systems with a bound state at zero energy. The eigenvalues
of the linearised renormalisation group are used to assign a systematic
power-counting to terms in the potential near each of these fixed points. The
expansion around the nontrivial fixed point is shown to be equivalent to the
effective-range expansion.
