1. Lars V. Ahlfors, Complex Analysis, 2nd ed., McGraw-Hill, New York, 1966.
2. Harold T. Davis, The Theory of Linear Operators from the Standpoint of Differential Equations of Infinite Order, Principia Press, Bloomington, Ind., 1936.
3. A. Erdélyi, Asymptotic Expansions, Dover, New York, 1965.
4. G. H. Hardy, Divergent Series, Oxford, 1949.