[1] S. P. Hastings, A. C. Lazer: 
On the Asymptotic Behavior of Solutions of the Differential Equation $y\sp{(4)} = p(t) y$. Czechoslovak Math. J., Vol. 18 (93) (1968) pp. 224-229. 
MR 0226110 | 
Zbl 0157.14901[3] J. W. Macki, J. S. Muldowney: 
The Asymptotic Behavior of Solutions to Linear Systems of Ordinary Differential Equations. Pacific J. Math., Vol. 33 (1970) pp. 693 - 706. 
DOI 10.2140/pjm.1970.33.693 | 
MR 0268463