Time-dependent Displaced and Squeezed Number States

We generalize the wave functions of the displaced and squeezed number states, found by Nieto, to a time-dependent harmonic oscillator with variable mass and frequency. These time-dependent displaced and squeezed number states are obtained by first squeezing and then displacing the exact number states and are exact solutions of the Schr\"{o}dinger equation. Further, these wave functions are the time-dependent squeezed harmonic-oscillator wave functions centered at classical trajectories.