1. L. Bieberbach, Analytic Continuation [in Russian], Moscow (1967).
2. L. Cattabianchi, ?Some criteria of non-continuability for power series,? Riv. Matem., Univ. Parma,3, No. 2, 169?91 (1962).
3. M. Cugiani, ?Variations of the conditioned sign, ?reduction? and the Fabry-Polya theorem,? Riv. Matem. Univ. Parma, Nos. 1?3, 99?132 (1957?58).
4. M. E. Noble, ?A note on non-continuable power series,? J. London Mathem. Soc.,35, No. 1, 117?127 (1960).
5. G. Ricci, ?Variations of the conditioned sign and the Fabry theorem,? Ann. Matem. Pura ed Appl.,38, 1?31 (1955).