[1] M. Háčik:
A note on higher monotonicity properties of certain Sturm-Liouville functions. Arch. Math. (Brno) XVI (1980), 153 -160.
MR 0594460
[2] M. Háčik:
Some analogues for higher monotonicity of the Sonin - Butlewski-Polya theorem. Mathematica Slovaca 31 (1981), 291-296.
MR 0621919
[3] L. Lorch M. E. Muldoon, P. Szego:
Higher monotonicity properties of certain Sturm-Liouville functions IV. Canad. J. of Math., Vol. XXIV (1972), 349-368.
MR 0298113