How to find polynomial coefficient

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Giancarlo Mariani 2017-12-27

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https://ww2.mathworks.cn/matlabcentral/answers/374664-how-to-find-polynomial-coefficient

回答： Sudheer Nuggehalli 2018-1-2

Hello,

I've a set of data x and y that give a curve and I wanna find an approximation of this curve using the form y=a1*x+a3*x^3. Using the function polyfit I can obtain a cubic form y=a0+a1*x+a2*x^2+a3*x^3, but I need a0 and a2 equal to 0. How can i do?

Thank you in advance.

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Answer 1

Sudheer Nuggehalli 2018-1-2

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https://ww2.mathworks.cn/matlabcentral/answers/374664-how-to-find-polynomial-coefficient#answer_298451

One possible way to go about doing this is to use the "fit" function, which creates the fit to the data in x and y with any custom model. The following code can be used as a starting point:

ft = fittype( @(a1,a3,x) a1*x+a3*x.^3 )

f = fit(x,y,ft)

The "fit" function requires an input argument object of type "fittype", which is created using the "fittype" function. The documentation pages below explain in greater detail how to use the "fit" and "fittype" functions, as well as detailed examples:

fit

fittype

I hope this helps!