A method to construct the nonlocal symmetries of nonlinear evolution equations

A method is proposed to seek the nonlocal symmetries of nonlinear evolution equations. The validity and advantages of the proposed method are illustrated by the applications to the Boussinesq equation, the coupled Korteweg-de Vries system, the Kadomtsev - Petviashvili equation, the Ablowitz - Kaup - Newell - Segur equation and the potential Korteweg-de Vries equation. The facts show that this method can obtain not only the nonlocal symmetries but also the general Lie point symmetries of the given equations.