Dudkin, M. Y. (2004). Point spectrum of the schrödinger operator with point interactions at the vertices of regular <em class="a-plus-plus">N</em&gt;-gons. Ukrains’kyi Matematychnyi Zhurnal, 56(8), 1128–1134. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/3827