Generalization of the Luttinger theorem for fermionic ladder systems

We apply a generalized version of the Lieb-Schultz-Mattis theorem to fermionic ladder systems to show the existence of a low-lying excited state (except for some special fillings). This can be regarded as a nonperturbative proof for the conservation under interaction of the sum of the Fermi wave vectors of the individual channels, corresponding to a generalized version of the Luttinger theorem to fermionic ladder systems. We conclude by noticing that the Lieb-Schultz-Mattis theorem is not applicable in this form to show the existence of low-lying excitations in the limit that the number of legs goes to infinity, e.g., in the limit of a 2D plane. [S0163-1829(98)04536-6].