Akhtyamov, A., V. Sadovnichy, and Y. Sultanaev. “Generalizations of Borg’s Uniqueness Theorem to the Case of Nonseparated Boundary Conditions”. Eurasian Mathematical Journal, vol. 3, no. 4, Dec. 2012, pp. 10-22, https://emj.enu.kz/index.php/main/article/view/803.