ZHANG, Ruda July 17, 2022:  Notes on perturbation theory. [27d4e84]
ZHANG, Ruda December 19, 2021:  Minor edits. [f64ba1c]
ZHANG, Ruda October 09, 2021:  Format math [70cc87b]
ZHANG, Ruda February 25, 2021:  Add perturbation theory for linear operators. [3aae292]