The expansion (21)2121p is equal to the rational p-adic number.
In the set of 3-adic numbers, the closed ball of radius 1/3 "centered" at 1, call it B, is the set . This closed ball partitions into exactly three smaller closed balls of radius 1/9: , , and . Then each of those balls partitions into exactly 3 smaller closed balls of radius 1/27, and the sub-partitioning can be continued indefinitely, in a fractal manner.
Likewise, going upwards in the hierarchy, B is part of the closed ball of radius 1 centered at 1, namely, the set of integers. Two other closed balls of radius 1 are "centered" at 1/3 and 2/3, and all three closed balls of radius 1 form a closed ball of radius 3, , which is one out of three closed balls forming a closed ball of radius 9, and so on.