1. Ahlbrandt, C.D.: Principal and antiprincipal solutions of self-adjoint differential systems and their reciprocals. Rocky Mn. J. Math. 2, 169–182 (1972)
2. Bernstein, D.S.: Matrix Mathematics. Theory, Facts, and Formulas with Application to Linear Systems Theory. Princeton University Press, Princeton (2005)
3. Campbell, S. L., Meyer, C. D.: Generalized Inverses of Linear Transformations Reprint of the: corrected reprint of the 1979 original, Classics in Applied Mathematics, vol. 56. Society for Industrial and Applied Mathematics (SIAM), Philadelphia, PA (1991). 2009
4. Clark, S., Gesztesy, F., Nichols, R.: Principal solutions revisited preprint (2014), arxiv:1401.1285v2
5. Coppel, W.A.: Disconjugacy Lecture Notes in Mathematics. Springer, Berlin (1971)