1. M. Verde:Nuovo Cimento,28, 547 (1963).
2. Dunford-Schwartz:Linear Operators, part I (New York, 1958), p. 614.
3. Dunford-Schwartz:Linear Operators, part I (New York, 1958), p. 566.
4. M. A. Naimark:Normed Rings (Amsterdam, 1959), p. 117.
5. A. Sommerfeld:Partial Differential Equations in Physics (New York, 1919), p. 233.