1. G. Darboux, “Sur une proposition relative aux equations lineaires,” C. R. Acad. Sci. Paris, 94, 1456–1459 (1882).
2. M. M. Crum, “Associated Sturm—Liouville systems,” Quart. J. Math., 6, 121–127 (1955).
3. V. B. Matveev and M. A. Salle, Darboux Transformations and Solitons, Springer, Berlin (1991).
4. A. B. Shabat, “Dressing chains and lattices,” in: Nonlinearity, Integrability, and All That: Twenty Years After NEEDS’79 (Lecce, Italy, 1–10 July 1999, M. Boiti, L. Martina, F. Pempinelly, B. Prinary, and G. Soliani, eds.), World Scientific, Singapore (2000), pp. 341–342.
5. D. Levi and R. Benguria, “Bäcklund transformations and nonlinear differential difference equations,” Proc. Nat. Acad. Sci. USA, 77, 5025–5027 (1980).