Dymarskii, Y. M. “On Manifolds of Eigenfunctions and Potentials Generated by a Family of Periodic Boundary-Value Problems”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 48, no. 6, June 1996, pp. 771-8, https://umj.imath.kiev.ua/index.php/umj/article/view/5277.