Disconjugacy and the rotation number for linear, non-autonomous Hamiltonian systems

with Roberta Fabbri and Russell Johnson,
Ann. Mat. Pura App.
185 (2006), S3-S21.

The rotation number for non-autonomous Hamiltonian systems is used to characterize the
weak disconjugacy property for such systems. The argument functions of Lidskii-Yakubovich
are an important tool in the proofs. It is shown that weakly disconjugate systems admit
principal solutions.