ABSTRACT
R eca l l that in the Gaussian approximation d iscussed
in Chapter III, the spin fluctuation [i. e. , the deviation of
the spin configuration CJ(x) f ro m the most probable con-
figuration σ (χ ) ] was trea ted by the quadratic, or harm onic ,
approximation. The perturbation expansion is just an e x -
tension of the Gaussian approximation to include the an-
harmonic te rm s in 1C. The m ost probable configuration
σ(χ ) is the configuration that m in im izes 1C [ct] , as was
explained in Chapter III. F o r T > T c and h = 0, we have
5(x) = 0 so that σ - 5 = σ. The perturbation expansion
discussed in this chapter has assumed that σ = 0.
