SorokinaN. G. “On the Normal Solvability of the Tricomi Problem for an Equation of the Lavrentyev — Bitsadze Type”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 23, no. 1, Dec. 1970, pp. 34-42, https://umj.imath.kiev.ua/index.php/umj/article/view/8476.