AldashevS. A. 2011. “Eigenvalues and Eigenfunctions of the Gellerstedt Problem for the Multidimensional Lavrent?Ev?Bitsadze Equation”. Ukrains’kyi Matematychnyi Zhurnal 63 (6), 827-32. https://umj.imath.kiev.ua/index.php/umj/article/view/2765.