Past exam of the advanced quantum theory: path integrals and many-particle physics course of the University of Oxford / 2023 / 1 / a / i

Assume $Q\ge 2$ and $L\ge 2$; the trivial exceptions are described below. In this periodic Potts chain, every satisfied bond contributes $-J$ to the energy, and no bond can contribute less. All $L$ bonds can be satisfied simultaneously by choosing the same state at every site. Conversely, satisfying every bond forces this uniform configuration. Therefore the ground state energy and degeneracy factor areThe $Q$ choices are distinct configurations on the labelled lattice: a global relabelling of the states is a symmetry, not an identification of microstates.