[R] nonlinear modeling with rational functions?

[Spencer Graves]

      Rational functions (ratios of polynomials) often provide good 
approximations to many functions.  Does anyone know of any literature on 
nonlinear modeling with rational functions, sequential estimation, 
diagnostics, etc.?  I know I can do it with "nls" and other nonlinear 
regression functions, but I'm wondering what literature might exist 
discussing how a search for an appropriate rational approximation? 

      Thanks,
      Spencer Graves