“…Now by applying Propositions 4 and 4 for the optimal system (4.47), we want to find all nonequivalent equations in the form of Eq. (1.1) admitting E-extensions of the principal Lie algebra L E , by one dimension, i.e, equations of the form (1.1) such that they admit, together with the one basic operator ∂ ∂t of L 1 , also a second operator X (2) . In each case which this extension occurs, we indicate the corresponding coefficients E, H and the additional operator X (2) .…”