1. Keldysh,~M.V. On some cases of  degeneracy of elliptic equations on the boundary of a~domain. Dokl. Akad. Mauk SSSR. 1951. Vol.~77, No.~2. P.~181--183.
Келдыш~М.В. О некоторых случаях вырождения уравнений эллиптического типа. Доклады АН СССР. 1951. Т.~77, №~2. С.~181--183.

2. Fichera, G. On a unified theory of boundary value problems for elliptic-parabolic equations of second order.
In Boundary Problems. Differential Equations; University of Wisconsin Press: Madison, Wisconsin, 1960; pp. 97--120.

3. Oleinik, O.A.; Radkevich, E.V. Second-Order Equations with Nonnegative Characteristic Form; Providence: Amer. Math. Soc., 2012.
Oleinik, O.A.; Radkevich, E.V. Equations with Non-Negative Characteristic Form; Moscow State University: Moscow, Russia, 2010. (In Russian)

4. Vragov, V.N. On the theory of boundary value problems for mixed-type equations. Differ. Equ. 1976, 13, 1098--1105. (In Russian)

5. Egorov, I.E.; Fedorov, V.E. Higher-Order Nonclassical Equations of Mathematical Physics. Computer Center of the SB RAS: Novosibirsk, 1995. (In Russian)
Nonclassical Highest Order Equations of Mathematical Physics; Computational Center of SB RAS: Novosibirsk, Russia, 1995. (In Russian)

6. Egorov, I.E.; Fedorov, V.E. Smooth solutions of parabolic equations with changing time direction. AIP Conf. Proc. 2018, 2048, 040013.

7. Kozhanov, A.I. Nonlocal Integro--Differential Equations of the Second Order with Degeneration. Mathematics, 2020, 8(4), 606.

8. Sobolev~S.L. Some Applications of Functional Analysis in Mathematical Physics, Providence: Amer. Math. Soc., 1991.

9. Ladyzhenskaya, O.A.; Ural'tseva, N.N. Linear and Quasilinear Elliptic Equations. Academic Press: New York, NY, USA, 1968.

10. Triebel, H. Interpolation Theory. Function Spaces. Differential Operators; VEB Deutscher Verlag der
Wissenschaften: Berlin, Germany, 1978.

11. Naimark,~M.A. Linear Differential Operators; Frederick Ungar: New York, 1968.
Наймарк, М.А. Линейные дифференциальные операторы. М.: Наука, 1969.

12. Kozhanov,~A.I. Boundary value problems for some classes of higher-order equations that are unsolved with respect to the highest derivative. Sib.  Math.~J. 1994. Vol.~35, No.~2, 324--340.
Кожанов, А.И. О краевых задачах для некоторых классов уравнений высокого порядка, неразрешенных относительно старшей производной. Сиб. мат. журн. 1994. Т.~35, №~2, 359--376.

13. Favini,~A., Yagi,~A. Degenerate Differential Equations in Banach Spaces. New York, Basel, Hong Kong: Marcel Dekker, Inc., 1999.

14. Demidenko, G.V., Uspenskii, S.V. Partial Differential Equations and Systems Not Solvable with Respect to Highest
Order Derivatives; Marcel Dekker: New York, NY, USA; Basel, Switzerland, 2003.

15. Sviridyuk, G.A., Fedorov, V.E. Linear Sobolev Type Equations and Degenerate Semigroups of Operators; VSP:
Utrecht, The Netherlands; Boston, MA, USA, 2003.

16. Zamyshlyaeva,~A.A. Sobolev Type Linear Equations of Higher Order; Chelyabinsk: Yuzhno-Uralsk. Gos. University, 2012.
Замышляева, А.А. Линейные уравнения соболевского типа высокого порядка. Челябинск: изд. Южно--Уральского госуниверситета. 2012.

17. Zamyshlyaeva,~A.A. Study of Sobolev Type Linear Models of Higher Order; Dis. Dokt. Fiz.-Mat. Nauk, Chelyabinsk, 2013.
Замышляева, А.А. Исследование линейных моделей соболевского типа высокого порядка. Дисс. д.ф.-м.н. Челябинск, 2013.

18.  Zhegalov,~V.I., Mironov,~A.N., Utkina,~E.A. Equations with Dominating Partial Derivative, Kazan: Kazan Federal University, 2014.
Жегалов,~В.И., Миронов,~А.Н., Уткина,~Е.А. Уравнения с доминирующей частной производной. Изд. Казанского федерального университета. 2014.

19. Kozhanov, A.I. Boundary value problems for a class of nonlocal integro-differential equations with
degeneration. Bull. Samara University Ser. Nat. Sci. 2017, 23, 19-24. (In Russian) [CrossRef]

20. Evans, L.C. Partial Differential Equations; American Mathematical Society: Providence, RI, USA, 1998.