L. P. Nizhnik and S. Albeverio, “Schrödinger operators with a number of negative eigenvalues equal to the number of point interactions”, MFAT, vol. 9, no. 4, pp. 273–286, Dec. 2003, Accessed: Feb. 04, 2026. [Online]. Available: https://zen.imath.kiev.ua/index.php/mfat/article/view/222