Insensitivity of Quantized Hall Conductance to Disorder and Interactions
  A two-dimensional quantum Hall system is studied for a wide class of potentials including single-body random potentials and repulsive electron-electron interactions. We assume that there exists a non-zero excitation gap above the ground state(s), and then the conductance is derived from the linear perturbation theory with a sufficiently weak electric field. Under these two assumptions, we proved that the Hall conductance $\sigma_{xy}$ and the diagonal conductance $\sigma_{yy}$ satisfy $|\sigma_{xy}+e^2\nu/h|\le{\rm const.}L^{-1/12}$ and $|\sigma_{yy}|\le{\rm const.}L^{-1/12}$. Here $e^2/h$ is the universal conductance with the charge $-e$ of electron and the Planck constant $h$; $\nu$ is the filling factor of the Landau level, and $L$ is the linear dimension of the system. In the thermodymanic limit, our results show $\sigma_{xy}=-e^2\nu/h$ and $\sigma_{yy}=0$. The former implies that integral and fractional filling factors $\nu$ with a gap lead to, respectively, integral and fractional quantizations of the Hall conductance. 
Tohru Koma