1. N.G. de Bruijn and P. Erdös. On a recursion formula and on some Tauberiantheorems. Submitted to J. Research Nat. Bur. Standards.
2. A property of power series with positive coefficients;Erdös;Bull. Am. Math. Soc.,1949
3. Aufgaben und Lehrsätze aus der Analysis, vol. 1;Pólya,1945