D. Korotkin, H. Nicolai, “The ernst equation on a riemann surface”, Nuclear Physics B, 429:1 (1994), 229
Л. В. Рыжик, Е. И. Шульман, “Об алгебре симметрий нелинейных интегрируемых уравнений”, ТМФ, 95:1 (1993), 34–41 ; L. V. Ryzhik, E. I. Shulman, “Symmetry algebra of nonlinear integrable equations”, Theoret. and Math. Phys., 95:1 (1993), 387–392
P. G. Grinevich, Springer Series in Nonlinear Dynamics, Nonlinear Processes in Physics, 1993, 108