Generalized effective-range expansion

A systematic and accurate procedure has been developed for calculating the coefficients phi((in/out))(ln) of the series expansion f(l)((in/out))(k) = Sigma(infinity)(n=0)(k-k(0))(n) phi((in/out))(ln) of the Jost functions in the vicinity of an arbitrary point k(0) in the complex momentum plane. This makes it possible to obtain an analytic expression for the S-matrix s(l)(k) = f(l)((out))(k)/f(l)((in))(k) around k(0) and locate its possible poles. In the particular case of l = 0 and k(0) = 0, any number of the parameters of the standard effective-range expansion k cot delta(0) = -1/a + r(0)k(2)/2 - ... are easily obtained using the corresponding coefficients phi((in/out))(ln). Numerical examples demonstrate the stability and accuracy of the proposed method.