On SK and KK Integrable Systems

To obtain new integrable nonlinear differential equations there are some well-known methods such as Lax equations with different Lax representations. There are also some other methods which are based on integrable scalar nonlinear partial differential equations. We show that some systems of integrable equations published recently are the ${\cal M}_{2}$-extension of integrable scalar equations. For illustration we give Korteweg-de Vries, Kaup-Kupershmidt, and Sawada-Kotera equations as examples. By the use of such an extension of integrable scalar equations we obtain some new integrable systems with recursion operators. We give also the soliton solutions of the system equations and integrable standard nonlocal and shifted nonlocal reductions of these systems.