Abstract:
Necessary conditions for the existence of nontrivial conservation laws for systems of
nonlinear equations of the form
$u_t=a(u,v)u_{xx}+b(u,v)v_{xx}+f(u,v,u_x,v_x),$$-v_t=c(u,v)u_{xx}+d(u,v)v_{xx}+g(u,v,u_x,v_x)$.
are found. They take the form of densities of local conservation laws constructed in
a definite manner from the coefficients of the system. The conditions can be readily
verified in each specific case. A module of simple invertible substitutions that makes
it possible to reduce the system to a canonical form when some integrability conditions
are satisfied is discussed.