  A fast community detection algorithm based on a q-state Potts model is
presented. Communities in networks (groups of densely interconnected nodes that
are only loosely connected to the rest of the network) are found to coincide
with the domains of equal spin value in the minima of a modified Potts spin
glass Hamiltonian. Comparing global and local minima of the Hamiltonian allows
for the detection of overlapping (``fuzzy'') communities and quantifying the
association of nodes to multiple communities as well as the robustness of a
community. No prior knowledge of the number of communities has to be assumed.
