[1] Hartman Philip: On Differential Equations and the Functions $J_{\mu}^2 +Y_{\mu}^2$. (Amer. Journ. of Math. Vol. 83, 1961, str. 154-187) MR 0123039

[2] Lorch Lee, Szego Peter: Higher Monotonicity Properties of Certain Sturm-Liouville Functions. (Acta Math. 109, 1963, str. 55-73) MR 0147695 | Zbl 0113.27704

[3] Lorch Lee, Szego Peter: Higher Monotonicity Properties of Certain Sturm-Liouville Functions II. (Bull. de l'Academie Polonaise des Sci. Ser. Math. Astr. et Phys. Vol. XI, 1963, str. 455-457). MR 0147695 | Zbl 0113.27704