  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.
