II. Correlation Functions and Universality

Correlation Function Ideas are explored using the two-dimensional Ising Model as an example Operator Algebras/Short Distance Expansions are explored as a technique for discussing invariance properties of critical models, including scale and conformal invariance. The effect of marginal operators is considered in detail. An explicit form for the spin correlation functions of the two dimensional Ising model at criticality is exhibited. This is then used to show the non-existence of marginal operators coupled to the spin. These operators would make the spin correlations non-universal. Universality is then proven for this model in a manner which is "good enough for physicists work..."