  Using linear invariant operators in a constructive way we find the most
general thermal density operator and Wigner function for time-dependent
generalized oscillators. The general Wigner function has five free parameters
and describes the thermal Wigner function about a classical trajectory in phase
space. The contour of the Wigner function depicts an elliptical orbit with a
constant area moving about the classical trajectory, whose eccentricity
determines the squeezing of the initial vacuum.
