Bringing the function to the special form

HI,

I have a fuction f(x,y) determined on all field x and y. I want transform it in ( simplified for example) y=x+R1^2+R^4+R^6, where Rn is coefficient that i dont know. It is enough for me to get coefficients for 10-20 points off f(x,y). Should i write a sycle or exist MathLab tool for that purpose?

Thank you

[Walter Roberson]

polyfit() ?

[Pavel Aksentsev]

No. I had that equation f(x,y) photo ( here I know all)

And I want get equation f(x,y) photo 2 ( here z and r are equivalent to x and y, I dont know alpfa1, alpfa2, alpfan, )

P.S. ( I made mistake in my example)))

Thank you