Polyfit with other model

Hi,

i am new here,

When i get it right polyfit is for polynomials like this: P(a) = c0 * a^0 + c1 * a^1 + c2 * a^2....

But how to get the coefficients from P(a) = 1 + c1 * a^1 + c^* a^2 .... So that the first value is fixed.

I only find this

https://www.mathworks.com/matlabcentral/answers/399683-polyfit-with-odd-powers-only

Do I just have to subtract from A "1" and from the y too and that's it?

(A and y from the link)

greetings

Answer 1

[Bruno Luong]

Generate test data

x=linspace(0,1,100)
P = [rand(1,3) 1]
y=polyval(P,x) + 0.1*randn(size(x));

Fit polynomial (order n) y=P(x) assuming cst-term = 1.

n = 3;
M = x(:).^(n-1:-1:1);
P = [M\(y(:)-1); 1];

Check

plot(x,y,'.');
hold on
plot(x,polyval(P,x),'r');