with the sum running over the nearest neighbor pairs ⟨ ⟨ i , j ⟩ ⟩ over all lattice sites, and J c is a coupling constant, determining the interaction strength. This model is now known as the vector Potts model or the clock model. Potts provided the location in two dimensions of the phase transition for q = 3 , 4 . In the limit q → → ∞ ∞ , this becomes the XY model.