This program calculates the fractional level populations of a 5-level
O II model atom, accounting only for collisional
excitation/de-excitation and spontaneous emission. A nice example of
solving a linear system of equations using matrix inversion.

REFERENCES:

"Astrophysics of Gaseous Nebulae and Active Galactic Nuclei" by
Osterbrock and Ferland (2nd ed.)