  We investigate the linear ac-conductance for tunneling through an arbitrary
interacting quantum dot in the presence of a finite dc-bias. In analogy to the
well-known Meir-Wingreen formula for the dc case, we are able to derive a
general formula for the ac-conductance. It can be expressed entirely in terms
of local correlations on the quantum dot, in the form of a Keldysh block
diagram with four external legs. We illustrate the use of this formula as a
starting point for diagrammatic calculations by considering the ac-conductance
of the noninteracting resonant level model and deriving the result for the
lowest order of electron-phonon coupling. We show how known results are
recovered in the appropriate limits.
