The 
model, for an integer 
, is a spin
model with unit vectors 
and interactions invariant under simultaneous
orthogonal transformations. For nearest-neighbor
edges 
of a finite square lattice, its
zero-field ferromagnetic Gibbs measure is proportional
to
 |
where 
is normalized surface measure
and each edge is counted once. The parameter 
is inverse temperature. The
cases 
and 
give the Ising model and
XY model, respectively.
