Y. A. Prykarpatsky and A. M. Samoilenko, “The classical M. A. Buhl problem, its Pfeiffer – Sato solutions and the classical Lagrange – D’Alembert principle for the integrable heavenly type nonlinear equations”, Ukr. Mat. Zhurn., vol. 69, no. 12, pp. 1652-1689, Dec. 2017.