Towards classification of quasi-local symmetries of evolution equations

We develop efficient group-theoretical approach to the problem of classification of evolution equations that admit non-local transformation groups (quasi-local symmetries), i.e., groups involving integrals of the dependent variable. We classify realizations of two- and three-dimensional Lie algebras leading to equations admitting quasi-local symmetries. Finally, we generalize the approach in question for the case of an arbitrary system of evolution equations with two independent variables.