Consider a set Λ Λ of lattice sites, each with a set of adjacent sites (e.g. a graph) forming a d -dimensional lattice. For each lattice site k ∈ ∈ Λ Λ there is a discrete variable σ σ k such that σ σ k ∈ ∈ { − − 1 , + 1 } , representing the site's spin. A spin configuration, σ σ = { σ σ k } k ∈ ∈ Λ Λ is an assignment of spin value to each lattice site.