The Living Thing / Notebooks :

(Weighted) least squares fits

The classic, surprisingly deep.

A few non-comprehensive notes here.

As used in, e.g. lasso regression.

Ceres Solve is an open source C++ library for modeling and solving large, complicated optimization problems. It can be used to solve Non-linear Least Squares problems with bounds constraints and general unconstrained optimization problems. It is a mature, feature rich, and performant library that has been used in production at Google since 2010.