___

• B G Konopelchenko, The two dimensioanal second-order differen- tial spectral problem: compatibility conditions, general Bts and integrable equations, Invers problems 4 (1988) 151-163
• C.Athorne, A Z2× R3Toda system, Phys. Lett.A 206 (1995) 162- 166
• S.P.Novikov and A.P.Veselov, Two-Dimensional Schrodinger opera- tor: Inverse Scattering Transform and Evolutional Equations, Phys- ica 18D (1986) 267-273