On the Yang-Baxter equation for the six-vertex model

In this paper we review the theory of the Yang-Baxter equation related to the 6-vertex model and its higher spin generalizations. We employ a 3D approach to the problem. Starting with the 3D R-matrix, we consider a two-layer projection of the corresponding 3D lattice model. As a result, we obtain a new expression for the higher spin $R$-matrix associated with the affine quantum algebra $U_q(\widehat{sl(2)})$. In the simplest case of the spin $s=1/2$ this $R$-matrix naturally reduces to the $R$-matrix of the 6-vertex model. Taking a special limit in our construction we also obtain new formulas for the $Q$-operators acting in the representation space of arbitrary (half-)integer spin. Remarkably, this construction can be naturally extended to any complex values of spin $s$. We also give all functional equations satisfied by the transfer-matrices and $Q$-operators.