The saga of the Ising susceptibility

We review developments made since 1959 in the search for a closed form for the susceptibility of the Ising model. The expressions for the form factors in terms of the nome $q$ and the modulus $k$ are compared and contrasted. The $λ$ generalized correlations $C(M,N;λ)$ are defined and explicitly computed in terms of theta functions for $M=N=0,1$.