Aldashev, S. A. “Existence of Eigenfunctions of the Tricomi Spectral Problem for Some Classes of Multidimensional Mixed hyperbolic–parabolic Equations”. Ukrains’kyi Matematychnyi Zhurnal 62, no. 6 (June 25, 2010): 723 -. Accessed April 27, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/2904.