Tetrahedron equations, boundary states and hidden structure of U_q(D_n¹)

Simple periodic 3d->2d compactification of the tetrahedron equations gives the Yang-Baxter equations for various evaluation representations of U_q(sl_n). In this paper we construct an example of fixed non-periodic 3d boundary conditions producing a set of Yang-Baxter equations for U_q(D_n^1). These boundary conditions resemble a fusion in hidden direction.