I am reading a paper related to rank-2 symmetric U(1) gauge theory: https://journals.aps.org/prb/abstract/10.1103/PhysRevB.98.035111 (or on arXiv: https://arxiv.org/abs/1802.10108). My question concerns a skipped calculation in Sec. II. There, the authors wrote that Aμν being compact (mod 2π) and the canonical commutator [Aμν,Eμν]=−i implies that the eigenvalues of Eμν are integers. I don't follow the authors reasoning here and I don't see why the eigenvalues should be integers. Would someone please enlighten me? Thanks!!