  Two well studied Ramsey-theoretic problems consider subsets of the natural
numbers which either contain no three elements in arithmetic progression, or in
geometric progression. We study generalizations of this problem, by varying the
kinds of progressions to be avoided and the metrics used to evaluate the
density of the resulting subsets. One can view a 3-term arithmetic progression
as a sequence $x, f_n(x), f_n(f_n(x))$, where $f_n(x) = x + n$, $n$ a nonzero
integer. Thus avoiding three-term arithmetic progressions is equivalent to
containing no three elements of the form $x, f_n(x), f_n(f_n(x))$ with $f_n
\in\mathcal{F}_{\rm t}$, the set of integer translations. One can similarly
construct related progressions using different families of functions. We
investigate several such families, including geometric progressions ($f_n(x) =
nx$ with $n > 1$ a natural number) and exponential progressions ($f_n(x) =
x^n$).
  Progression-free sets are often constructed "greedily," including every
number so long as it is not in progression with any of the previous elements.
Rankin characterized the greedy geometric-progression-free set in terms of the
greedy arithmetic set. We characterize the greedy exponential set and prove
that it has asymptotic density 1, and then discuss how the optimality of the
greedy set depends on the family of functions used to define progressions.
Traditionally, the size of a progression-free set is measured using the (upper)
asymptotic density, however we consider several different notions of density,
including the uniform and exponential densities.
