PRYKARPATSKY, Y. A.; SAMOILENKO, A. M. The classical M. A. Buhl problem, its Pfeiffer – Sato solutions and the classical Lagrange – D’Alembert principle for the integrable heavenly type nonlinear equations. Ukrains’kyi Matematychnyi Zhurnal, v. 69, n. 12, p. 1652-1689, 25 Dec. 2017.