[2] N. Dunford, J. T. Schwartz:
Linear operators. General theory, Interscience Publishers, New York, London, 1958.
MR 0117523
[3] I. Ličko, M. Švec: Le caractère oscillatoire des solutions de l’équation $y^{(n)}+f(t)y^\alpha =0$, $n>1$. Czech. Math. J. 13 (88), 481–491.
[4] M. Švec: Asymptotic equivalence and oscillatory properties of ordinary differential equations. Equadiff 78 (1978), 213–222.