1. H. Block, “Sur les equations lineaires aux derivees partielles a carateristiques multiples,” Ark. Mat., Astron., Fys. Note 1, 7, No. 13, 1–34 (1912); Ark. Mat., Astron., Fys. Note 2, 7, No. 21, 1–30 (1912); Ark. Mat., Astron., Fys. Note 3, 8, No. 23, 1–51 (1912–1913).
2. L. Cattabriga, “Una generalizationi del problema fundamentale di valori al contorno per eguationi paraboliche lincari,” Ann. Mat. Pura Appl., 46, 215–247 (1958).
3. L. Cattabriga, “Potenziali di linea e di dominio per equazioni non paraboliche in due variabili a caratteristiche multiple,” Rend. Semin. Mat. Univ. Padova, 31, 1–45 (1961).
4. S. Abdinazarov, Boundary-Value Problems for Equations with Multiple Characteristics [in Russian], Doctoral-Degree Thesis (Physics and Mathematics), Tashkent (1992).
5. T. D. Dzhuraev and Yu. P. Apakov, “On the self-similar solution of a third-order equation with multiple characteristics,” Vestn. Samar. Gos. Tekh. Univ., Ser. Fiz.-Mat. Nauk., No. 2(15), 18–26 (2007).