  A common tool in Casimir physics (and many other areas) is the asymptotic
(high-frequency) expansion of eigenvalue densities, employed as either input or
output of calculations of the asymptotic behavior of various Green functions.
Here we clarify some fine points and potentially confusing aspects of the
subject. In particular, we show how recent observations of Kolomeisky et al.
[Phys. Rev. A 87 (2013) 042519] fit into the established framework of the
distributional asymptotics of spectral functions.
