Hello Radovan, you will read these few samples
They all fit univariate wrt to 'x'
The bivariate [xy, z] is as simple as univariate.
For this case, I'm only aware of ordinary polynomials. It does not
exclude orthogonal polynomials.
The complete Treasury suggests an Electronic Book [Mathsoft style]

1. Chemist is not fittable in Mathcad, Smath as well, only by hand.
2. Lorentz Peak is 10 parameters.
3. Laser 10 parameters [lasted ~ 1 month Mathsoft Collaboratory].

Jean

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