A general formulation of least squares

Question

I am confused about of writing the formulation using linear least squares ,there is references writing it such as :

$$ ||Ax-b ||^2_2$$ and other references such as : $$ ||b-Ax ||^2_2$$ What is meant by each formulation?and when to use ?

Answer 1

They are totally identical since $\|{\bf v}\|=\|{-\bf v}\|$. It doesn't make any difference which you use.