A semi-discrete scheme for solving nonlinear hyperbolic-type partial integro-differential equations using radial basis functions A comparative analysis of Painlevé, Lax pair, and similarity transformation methods in obtaining the integrability conditions of nonlinear Schrödinger equations Chen, Lee, and Liu presented in 1979 an algorithm for establishing integrability of two-dimensional partial differential systems. It is proved here that this algorithm is invariant under the point transformations, differential substitutions, and some integrodifferential substitutions. It is also proved that canonical conserved densities of linearizable systems arising in the frameworks of the method are almost all trivial. The integrability of the non-Newtonian liquid equations is investigated and it is proved that there exist two integrable systems only. A preliminary classification of the third-order integrable evolution systems is presented. © 1997 American Institute of Physics.Here the matrices U and V are local functions of u ␣ and a complex parameter ; L, A, and B are the linear scalar differential operators depending on ץ x only ͑but not ץ t ͒. Coefficients of the L, A, and B are local functions of u ␣ and ͑see Zakharov et al., 16 for instance͒. Equation ͑3͒ is called the zero curvature representation of the system ͑1͒. Equation ͑4͒ is called the triadic representation of the system ͑1͒ or (L,A,B) representation. Let us consider the triadic representation for definiteness. It is obvious that an operator equation Hϭ0 is equivalent to its adjoint H ϩ ϭ0, therefore the equation ͑4͒ impliesEquations ͑4͒ and ͑5͒ provide the compatibility for the following linear systems: 6429 A. G. Meshkov: Integrodifferential substitutions If the system (1) is formally integrable, then the system (8) has a solution in the form of the formal Laurent expansions, 6430 A. G. Meshkov: Integrodifferential substitutions