[1] LORCH L., SZEGO P.: Higher monotonicity properties of certain Sturm-Liouville functions. Acta Math. 109, 1963, 55-73. MR 0147695 | Zbl 0113.27704

[2] LORCH L., MULDOON M. E., SZEGO P.: Higher monotonicity properties of certain Sturm-Liouville functions III. Can. J. Math. 22, 1970, 1238-1265. MR 0274845 | Zbl 0212.40703

[3] LORCH L., MULDOON M. E., SZEGO P.: Higher monotonicity properties of certain Sturm-Liouville functions IV. Can. J. Math. 24, 1972, 349-368. MR 0298113 | Zbl 0242.34030