BerezanskyYu. M., and GekhtmanM. I. 1990. “Inverse Problem of Spectral Analysis and the Non-Abelian Chains of Nonlinear Equations ”. Ukrains’kyi Matematychnyi Zhurnal 42 (6), 730-47. https://umj.imath.kiev.ua/index.php/umj/article/view/8829.