  Algebraic topology studies topological spaces with the help of tools from
abstract algebra. The main focus of this paper is to show that many concepts
from algebraic topology can be conveniently expressed in terms of (normal)
factor graphs. As an application, we give an alternative proof of a classical
duality result of Kramers and Wannier, which expresses the partition function
of the two-dimensional Ising model at a low temperature in terms of the
partition function of the two-dimensional Ising model at a high temperature.
Moreover, we discuss analogous results for the three-dimensional Ising model
and the Potts model.
