[1]
N. Bilusyak, L. I. Komarnyts’ka, and B. I. Ptashnik, “Dirichlet-Type Problems for Systems of Partial Differential Equations Unresolved with Respect to the Highest Time Derivative”, Ukr. Mat. Zhurn., vol. 54, no. 12, pp. 1592-1602, Dec. 2002.