Although [Vitor Fróis] is explaining linear regression because it relates to machine learning, the post and, indeed, the topic have wide applications in many things that we do wi ...

Arthur Albert, The Gauss-Markov Theorem for Regression Models with Possibly Singular Covariances, SIAM Journal on Applied Mathematics, Vol. 24, No. 2 (Mar., 1973), pp. 182-187 ...