Fisher zeros and singular behaviour of the two-dimensional Potts model in the thermodynamic limit

The duality transformation is applied to the Fisher zeros near the ferromagnetic critical point in the q>4 state two-dimensional Potts model. A requirement that the locus of the duals of the zeros be identical to the dual of the locus of zeros in the thermodynamic limit (i) recovers the ratio of specific heat to internal energy discontinuity at criticality and the relationships between the discontinuities of higher cumulants and (ii) identifies duality with complex conjugation for the zeros near the ferromagnetic critical point. The conjecture that all zeros governing ferromagnetic singular behaviour satisfy the latter requirement gives the full locus of such Fisher zeros to be a circle. This locus, together with the density of zeros is then shown to be sufficient to recover the singular part of the thermodynamic functions in the thermodynamic limit, their regular parts coming from separate loci of zeros not crossing the positive real temperature axis.