where the FK clusters are the connected components of the union of closed segments ∪ ∪ ( i , j ) ∈ ∈ ω ω [ i , j ] . This is proportional to the partition function of the random cluster model with the open edge probability p = v 1 + v = 1 − − e − − J p . An advantage of the random cluster formulation is that q can be an arbitrary complex number, rather than a natural integer.