Differential substitutions for vectorial generalizations of the mKdV equation (Q887491)

The author considers differential substitutions of the form \(u=\sum_{i=0}^m\xi_iv_i\) for vectorial generalizations of the mKdV equation: \(u_t=u_3+(u,u)u_1\) and \(u_t=u_3+(u,u)u_1+(u,u_1)u_1\). It appears that only substitutions of the form \(u=F(v,v_x)\) are admitted.