where "a" is a non-negative integer and "n" is an integer in "pa". For example, in the dyadic integers, "U"1(1) is the set of odd numbers. "Ua"("n") is the set of all "p"-adic integers whose difference from "n" has "p"-adic absolute value less than "p"1−"a". Then is a compactification of , under the derived topology (it is "not" a compactification of with its usual discrete topology). The relative topology on as a subset of is called the -adic topology on .