Stephen A. Fenner, Lance J. Fortnow, and Stuart A. Kurtz. Gap-definable counting classes. Lance Fortnow and Nick Reingold. PP is closed under truth-table reductions. Computational complexity of probabilistic Turing machines.
Nondeterministic space is closed under complementation.
Ramamohan Paturi and Michael E. Saks. Approximating threshold circuits by rational functions. Charles W. Rackoff and Joel I. Seiferas. Limitations on separating nondeterministic complexity classes. Walter L. Ruzzo, Janos Simon, and Martin Tompa.