V. L. Vereshchagin, “Darboux-integrable discrete systems”, Theoret. and Math. Phys., 156:2 (2008), 1142–1153
O. V. Kaptsov, “Invariants of characteristics of some systems of partial differential equations”, Siberian Math. J., 45:3 (2004), 475–487
A. M. Gurieva, A. V. Zhiber, “Laplace Invariants of Two-Dimensional Open Toda Lattices”, Theoret. and Math. Phys., 138:3 (2004), 338–355
A. V. Zhiber, S. Ya. Startsev, “Integrals, Solutions, and Existence Problems for Laplace Transformations of Linear Hyperbolic Systems”, Math. Notes, 74:6 (2003), 803–811
A. V. Zhiber, V. V. Sokolov, “Exactly integrable hyperbolic equations of Liouville type”, Russian Math. Surveys, 56:1 (2001), 61–101
S. Ya. Startsev, “Hyperbolic Equations Admitting Differential Substitutions”, Theoret. and Math. Phys., 127:1 (2001), 460–470
S. P. Tsarev, “Factoring linear partial differential operators and the Darboux method for integrating nonlinear partial differential equations”, Theoret. and Math. Phys., 122:1 (2000), 121–133
S. Ya. Startsev, “Laplace invariants of hyperbolic equations linearizable by a differential substitution”, Theoret. and Math. Phys., 120:2 (1999), 1009–1018
V. E. Adler, S. Ya. Startsev, “Discrete analogues of the Liouville equation”, Theoret. and Math. Phys., 121:2 (1999), 1484–1495
S. P. Tsarev, “On Darboux-Integrable Nonlinear Partial Differential Equations”, Proc. Steklov Inst. Math., 225 (1999), 372–381