The normalized rational fraction ranks first to fit data.
There are some exception to this rule. The Mathcad/Mathsoft
is easy to use as it requires no guess values. It is powered
from its built-in "genfit" Levenberg-Marquardt. Smath does not
have a "genfit equivalent". Thus, for the rational fit we are
confined to play lottery for initialising the rational fraction.
This example works fine after winning the "lottery".

Jean

Genfit Rational Type J.sm (41 KiB) downloaded 52 time(s).