We give necessary and sufficient conditions for a sequence to be exactly
realizable as the sequence of numbers of periodic points in a dynamical system.
Using these conditions, we show that no non-constant polynomial is realizable,
and give some conditions on realizable binary recurrence sequences. Realization
in rate is always possible for sufficiently rapidly-growing sequences, and is
never possible for slowly-growing sequences. Finally, we discuss the
relationship between the growth rate of periodic points and the growth rate of
points with specified least period.Comment: Revised version with new content; to appear in J.Integer Sequence