Myshkis, A.A., et al. «Oscillatory and Asymptotic Properties of the Solutions of a Class of Operator-Differential Equations». Revista Matemática Complutense, vol. 7, n.º 2, janeiro de 1994, p. 309, https://doi.org/10.5209/rev_REMA.1994.v7.n2.17747.