Lavrent’evA. S. “A Singularly Perturbed Spectral Problem for a Biharmonic Operator With Neumann Conditions”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 51, no. 11, Nov. 1999, pp. 1467–1475, https://umj.imath.kiev.ua/index.php/umj/article/view/4748.