[1] F. V. Atkinson: 
Discrete and Continuous Boundary Probems. Academic Press, New York and London, 1964. 
MR 0176141 
[2] H. Buchholz: 
Die  Konfluente Hypergeometrische  Funktion. Eгgebnisse  deг Angewandten Mathematik, 2, Springer, Berlin, 1953. 
MR 0054783 | 
Zbl 0050.07402 
[3] A. Erdélyi, al: Higher Transcendental Functions. 3 volumes, McGraw-Hill Book Company, New York, 1953.
[5] R. P. Soni: 
On an inequality for modified Bessel functions. Jour. Math. and Physics 44, 406-407 (1965). 
MR 0185164 | 
Zbl 0137.04801 
[6] G. Szegő: Orthogonal Polynomials. Rev. Ed., Amer. Math. Soc. Colloq. Publications, Vol. 23, New Yoгk, 1959.