  We theoretically study the collective excitations of an ideal gas confined in
an isotropic harmonic trap. We give an exact solution to the Boltzmann-Vlasov
equation; as expected for a single-component system, the associated mode
frequencies are integer multiples of the trapping frequency. We show that the
expressions found by the scaling ansatz method are a special case of our
solution. Our findings, however, are most useful in case the trap contains more
than one phase: we demonstrate how to obtain the oscillation frequencies in
case an interface is present between the ideal gas and a different phase.
