Ceres Solver is an open source C++ library for modeling and solving
large, complicated optimization problems. It is a feature rich, mature
and performant library which has been used in production at Google
since 2010. Ceres Solver can solve two kinds of problems.

1. Non-linear Least Squares problems with bounds constraints.
2. General unconstrained optimization problems.

WWW: http://ceres-solver.org/