How to define a multivariable function (for instance an anonymous function) of a number of variables that varies according to the size of an array variable?

I have two vectors input and output ,where S is the number of samples

I want to solve this problem as nonlinear least-squares (nonlinear data-fitting), so I have to define my anonymous function as follow

,where L is the number of coefficients and X is the regression matrix of x

How to define vector of coefficients ?!

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[Stephen23]

"How to define vector of coefficients"

https://www.mathworks.com/help/matlab/learn_matlab/matrices-and-arrays.html

[Abdelwahab Afifi]

In my case I want to define vector t of variables

Answer 1

[Nicolas B.]

I would be you, I would use a vector input and then, you return a vector.

[Abdelwahab Afifi]

more illustration please