We identify the r-matrix governing the Poisson brackets of the matrix elements of the Lax operator of the bi-YB-WZ model.
IntroductionStrong integrability is the property of certain Lax integrable theories. It amounts to Poissoncommutativity of the integrals of motions which are obtained as the spectral invariants of the Lax operator. In this paper, we focus on the issue of the strong integrability of Lax integrable non-linear σ-models.The epitome of the Lax integrable non-linear σ model is the principal chiral model [45], which can be formulated on any quadratic Lie group. First examples of integrable deformations of the principal chiral model were obtained in Refs. [1,6,14] for the group SU (2) and it took one more decade to understand the algebraic structure behind those theories which permitted to generalize them to live on every simple compact group target. In particular, the models [6,14] were generalized in this way in Refs. [24][25][26] in the framework of so-called η-deformation (or Yang-Baxter deformation) procedure, while the model [1] was generalized in Ref. [43] in the framework of λ-deformations. Since then, many other developments have followed, proving the Lax integrability of complex structure-induced deformations [5], of homogeneous Yang-Baxter deformations [4,23,41], of deformations with a WZ term [8,13], of coset spaces [12], of combined η and λ deformations [44] or of coupled η and λ deformations [2,9,10,[16][17][18]35]. Moreover, it was established in Refs. [20,27,29,44], that some of those integrable deformations are related by Poisson-Lie T-duality [30].The strong integrability of the principal model was proved in Ref. [38], that of the single and of the double Yang-Baxter deformations [25,26] was established in [11,12] and that of the single Yang-Baxter deformation with WZ term was proved in [13]. On the other hand, the strong integrability of the λdeformed σ-model [43] was demonstrated in Ref. [21,22] and of the coupled λ-deformed models [16][17][18] in Ref. [19]. Finally, the construction of the Lax pairs and the proof of the strong integrability of the coupled η and λ deformations [9,10,35] was completed in Ref. [2].