Ground-state entropy of the Potts antiferromagnet on cyclic strip graphs

We present exact calculations of the zero-temperature partition function (chromatic polynomial) and the (exponent of the) ground-state entropy S₀ for the q-state Potts antiferromagnet on families of cyclic and twisted cyclic (Möbius) strip graphs composed of p-sided polygons. Our results suggest a general rule concerning the maximal region in the complex q plane to which one can analytically continue from the physical interval where S₀ > 0. The chromatic zeros and their accumulation set exhibit the rather unusual property of including support for Re(q) < 0 and provide further evidence for a relevant conjecture.