i want write the next function by matlab ?

[John D'Errico]

It is time for you to learn to use loops. Here a for loop is simple and reasonable.

[mohammed elmenshawy]

i write the following and displayed the error

. what to do ?

Answer 1

[Walter Roberson]

You syms Z but what is z (lower-case) ? Is it a function or is it a vector?

It is not possible to use a symbolic expression to index an array. If you have a finite vector to sum symbolically, then construct all elements of the vector and then sum() them. Constructing the elements of the vector is sometimes possible using vectorization.

t = 4:n;
L = sum((z(t) - null1 .* z(t-1) - null2 .* z(t-2) + theta .* alpha(t-1)).^2) ./ (2 .* sigma.^2);