Ising model
An Ising model on a graph has for its configuration space the set of all possible assignments of +1 or -1 to each vertex of the graph. There is a function, E(e) giving the difference between the energy of the "bond" associated with the edge when the spins on both ends of the bond are opposite than when they are aligned. It's also possible to have an external magnetic field.
At a finite temperature, T, the probability of a configuration is proportional to .
This is a model in statistical mechanics.
The Ising model undergoes a phase transition between an ordered and a disordered phase in 2 dimensions or more.
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