M. S. Salakhitdinov, A. Tolipov, “Certain boundary value problems for a certain class of equations of mixed type”, Differ. Uravn., 9:1 (1973), 142–148
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2.
M. S. Salakhitdinov, A. Tolipov, “Certain boundary value problems for equations of mixed type with one and two curves of degeneracy”, Differ. Uravn., 8:1 (1972), 134–142