A frequency domain approach for reducing linear, time-invariant systems is presented which produces stable approximations of stable systems. The method is based upon the Schwarz canonical form and is shown to have a continued-fraction representation. A link between this method and the popular Routh approximation is also given. Further, the Schwarz approximation is combined with a moments-matching technique to improve steady-state responses to step and polynomial inputs. Examples are given to illustrate the methods.
Lucas, T. N., & Davidson, A. M. (1983). Frequency-domain reduction of linear systems using Schwarz approximation. International Journal of Control, 37(5), 1167-1178. https://doi.org/10.1080/00207178308933038