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We discuss the density-density response functions and the collective excitation spectrum for the extended Hubbard model with on-site attraction and arbitrary electron density, in the superconducting ground state. For the short-range intersite interaction we find the sound wave-like modes with the velocity interpolating smoothly between the weak (v ∝ 2Zt, |U| ≪ 2Zt) and strong coupling (v ∝ Zt^{2}/|U|,|U|...
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