1. Barenblatt, G.I., Entov, V.M., and Ryzhik, V.M., The Theory of Nonstationary Filtration of Fluid and Gas, (Nedra,
Moscow, 1972) [in Russian].
2. Bautin, N.N. and Leontovich, E.A., Methods and
Rules for the Qualitative Study of Dynamical Systems on the Plane, (Nauka, Moscow,
1990) [in Russian].
3. G. V. Demidenko, “Quasielliptic operators and Sobolev type equations,” Sib.
Mat. Zh. 49, 1064 (2008) [Sib. Math. J. 49, 842 (2008)].
4. G. V. Demidenko, “Quasielliptic operators and equations not solvable with
respect to the higher order derivative,” Sibir. Zh. Chist. i Prikl. Mat. 16 (3), 15 (2016) [J. Math.
Sci. 230, 25 (2018)].
5. G. V. Demidenko and S. V. Uspenskii, Partial Differential Equations and Systems not Solvable with Respect to the
Highest-Order Derivative (Marcel Dekker, New York, Basel;
2003).