### 7.3 Mode calculations

Although the modes have been well studied there remain a few open issues. In the black hole case, the
Kerr–Newman spectrum is known only in a restricted case [124], so the general case should be studied,
probably by evolving the perturbation equations. For non-rotating stars, there remain certain technical
questions, i.e. to find if there exist a class of -modes with even larger imaginary part or the existence of
extra interface modes. But for rotating stars, even slowly rotating ones, there remains much work [192].
There should be estimates of the fluid and spacetime modes for slowly rotating stars for various realistic
equations of state, and the results should be incorporated into the method suggested in [19]
for the estimates of the stellar parameters. There should be work towards understanding the
possible interaction of -modes with -modes [176, 93]. And finally the time dependent
perturbation equations for rotating stars should be evolved, because in this way we expect
to see most of the new features that rotation induces in the spectra (splitting, instabilities
etc).