Aldashev, S. A. “Nonuniqueness of the Solution of the Gellerstedt Space Problem for One Class of Many-Dimensional Hyperbolic-Elliptic Equations”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 62, no. 2, Feb. 2010, pp. 265–269, https://umj.imath.kiev.ua/index.php/umj/article/view/2862.