Singularities in Negami's splitting formula for the Tutte polynomial

The n-sum graph Negami's splitting formula for the Tutte polynomial is not valid in the region (x−1)(y−1)=q for q=1,2,…n−1 with the additional region y=1 if n>3. This region corresponds to (up to prefactors and change of variables) the Ising model, the q-state Potts model, the number of spanning forest generator and particularizations of these. We show splitting formulas for these specializations.