Full powers in arithmetic progressions

For given positive integers a and n, we consider the three-term arithmetic progressions a(2), y(n), x(2), where x and y are unknown integers. We give explicit upper bounds both for the number of such arithmetic progressions and for max{\x\, \y\}. Moreover, we find all such progressions with 1 less than or equal to a less than or equal to 1000, and 3 < n < 80.