Vonsovskii S. V. Some types of instabilities on the electron energy spectrum of the polar model of the crystal : The criterion of stability of a metallic state / S. V. Vonsovskii, M. I. Katsnelson // J. Phys. C: Solid State Phys. – 1979. – V. 12, N 11. – P. 2055-2064.