Fitting a fourth order polynomial

Hell everybody, i am trying to fit my data. The theory says that it should be a fourth order polynomial (a * x^4 + c). I used the command fitobject = fit(x,y, 'poly4'), but obviously this does not work, because it gives me a function of the form (ax^4 + bx^3 + c^x^2 + dx + e). I also tried the command fit = polyfit(x,y,4); but this seems to produce the same outcome. So I am a little lost how to do such a fit in matlab.

Thank you for you help in advance!

Best regards!

Christopher

Answer 1

[Matt J]

You can use bounds to force the unwanted coefficients to zero.

lb=[-inf,0,0,0,-inf];

fitobject = fit(x,y, 'poly4','Lower',lb,'Upper',-lb);

[Christopher Peter Wutti]

Hello, thank you very much for the answers, this is wonderful, now I get a polynomial of the form a x^4 + bx + c, so is there a way to also fix the 'b' to 0 ?

[Torsten]

lb = [-Inf ,0, 0, 0, -Inf];

Matt forgot one "0".