Akhtyamov, A.M., V.A. Sadovnichy, and Ya.T. Sultanaev. 2012. “Generalizations of Borg’s Uniqueness Theorem to the Case of Nonseparated Boundary Conditions”. Eurasian Mathematical Journal 3 (4):10-22. https://emj.enu.kz/index.php/main/article/view/803.