Asymptotic behavior of eigenvalues for first-order systems with distributional coefficients (Q6114482)

The authors study the asymptotic behaviour of the eigenvalues of the problem \[ Ju'+q(\lambda)u=\lambda w u \] with separated boundary conditions when \(q\) and \(w\) are real-valued symmertric and finite signed continuous measures (possibly satisfying some further conditions; \(J\) is the constant \(2\times 2\) matrix with \(-J^2=I\)). Central is the use of a Prüfer angle which is subjected to a Kepler transformation. Here it would have been helpful if they had referred for example to \S2 of the book by \textit{B. M. Brown} et al. [Periodic differential operators. Basel: Birkhäuser (2013; Zbl 1267.34001)].