27.1 Root finding
27.1.8 Davidon–Fletcher–Powell (DFP) formula
https://en.wikipedia.org/wiki/Davidon%E2%80%93Fletcher%E2%80%93Powell_formula
27.1.9 Berndt–Hall–Hall–Hausman algorithm
https://en.wikipedia.org/wiki/Berndt%E2%80%93Hall%E2%80%93Hall%E2%80%93Hausman_algorithm