  A Wilsonian renormalisation group is used to study nonrelativistic two-body
scattering by a short-ranged potential. We identify two fixed points: 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.
