On the Singularities in the Susceptibility Expansion for the   Two-Dimensional Ising Model
  For temperatures below the critical temperature, the magnetic susceptibility for the two-dimensional isotropic Ising model can be expressed in terms of an infinite series of multiple integrals. With respect to a parameter related to temperature and the interaction constant, the integrals may be extended to functions analytic outside the unit circle. In a groundbreaking paper, B. G. Nickel identified a class of singularities of these integrals on the unit circle. In this note we show that there are no other singularities on the unit circle. 
Craig A. Tracy, Harold Widom
math-ph math.MP