A few Lie algebras and their applications for generating integrable hierarchies of evolution types

A Lie algebra consisting of 3 × 3 matrices is introduced, whose induced Lie algebra by using an inverted linear transformation is obtained as well. As for application examples, we obtain a unified integrable model of the integrable couplings of the AKNS hierarchy, the D-AKNS hierarchy and the TD hierarchy as well as their induced integrable hierarchies. These integrable couplings are different from those results obtained before. However, the Hamiltonian structures of the integrable couplings cannot be obtained by using the quadratic-form identity or the variational identity. For solving the problem, we construct a higher-dimensional subalgebra R and its reduced algebra Q of the Lie algebra A2 by decomposing the induced Lie algebra and then again making some linear combinations. The subalgebras of the Lie algebras R and Q   do not satisfy the relation (G=G1⊕G2,[G1,G2]⊂G2), but we can deduce integrable couplings, which indicates that the above condition is not necessary to generate integrable couplings. As for application example, an expanding integrable model of the AKNS hierarchy is obtained whose Hamiltonian structure is generated by the trace identity. Finally, we give another Lie algebras which can be decomposed into two simple Lie subalgebras for which a nonlinear integrable coupling of the classical Boussinesq–Burgers (CBB) hierarchy is obtained.

Research highlights
► Two kinds of integrable models are obtained.
► An expanding integrable model of the AKNS hierarchy is derived.
► A kind of nonlinear integrable couplings of the CBB is presented.