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