Magnetism and infrared divergence in a Hubbard-phonon interacting system

We show that a finite Hubbard-phonon interacting system has ferromagnetic or unique spin-singlet ground state under the infrared singular condition. The key tool is a unitary transformation introduced by Arai and Hirokawa. We construct a concrete infrared singular representation using the operator algebraic method. The method is essentially same as one for the van Hove model using the Wightman functional method.