Berezansky, Yu. M., and M. I. Gekhtman. 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.