1. Boundary value and expansion problems of ordinary linear differential equations;Birkhoff;Trans. Amer. Math. Soc.,1908
2. General boundary conditions for an ordinary linear differential system;Cole;Trans. Amer. Math. Soc.,1964
3. The Numerical Treatment of Differential Equations;Collatz,1960
4. Methods of Mathematical Physics, Vol. II, Partial Differential Equations;Courant,1962
5. Linear Operators, Part I, General Theory;Dunford,1958