will be seen to describe the interaction between nearest neighbors. Of course, different functions give different interactions; so a function of s 0, s 1 and s 2 will describe a next-nearest neighbor interaction. A function V gives interaction energy between a set of spins; it is not the Hamiltonian, but is used to build it. The argument to the function V is an element s ∈ Q, that is, an infinite string of spins. In the above example, the function V just picked out two spins out of the infinite string: the values s 0 and s 1. In general, the function V may depend on some or all of the spins; currently, only those that depend on a finite number are exactly solvable.