writing out the least square expression

Heya, I am trying to find some variables through the least squares method.

X^2 = SUM(( ( I(i) - I(a,b,K,T(i)))^2 )/Sigma(i)^2).

I am however not sure how to declare my variables. I have my experimental data I(i), my standard deviation Sigma(i) and T(i). But I want Matlab to use the same a, b, K for each iteration. And give a, b, K for which X^2 is minimised.

Could someone point me towards a function/the right syntax, and tell me how I should define the variables and constants?

Thanks a lot, Mark