Triplet order parameter of the triangular Ising model

Considers the usual Ising model with pair interactions between nearest-neighbour spins sigma on the triangular lattice. The author evaluates the correlation M3=( sigma 1 sigma 2 sigma 3), where sigma 1, sigma 2, sigma 3 are the three spins at the vertices of a triangular face. This can be thought of as an order parameter associated with a three-spin 'field'. It vanishes at and above Tc, and has a critical exponent of 1/8.