Closed states of HA ^x models in cube representation. The symbols correspond to those in Fig. 1. Each model in systematic representation in Fig. 1 was translated into the cube representation. The model structure follows a respective change of the dimension n_D. (A) HA² model containing four states arranged in a 2-D square. (B) HA³ model containing eight states arranged in a 3-D cube. (C) HA⁴ model containing 16 states arranged in a 4-D hypercube. (D) HA⁵ model containing 32 states arranged in a 5-D hypercube. The K_x are indicated apart from the HA⁵ model where they were omitted for clarity.