George D. Birkhoff and Rudolph E. Langer,
``The Boundary Problems and Developments Associated with a System of Ordinary Linear Differential Equations of the First Order,''
Proceedings of the American Academy of Arts and Sciences 58 49-128 (1923).
Reprinted in Collected Mathematical Papers of George David Birkhoff,
Dover Publications Inc., New York, 1968. pp.345-424.