Using set notation for sets with repeating characters Consider the following set:
{x, y, yy, yyyy, yyyyyyyy, yyyyyyyyyyyyyyyy, ...}
How would set builder notation be used to represent such a set? My understanding of the limitation is very limited as I only have experience using it for sets whose only elements are numbers, but never with character strings.
I also do not understand how to specify just one starting character that doesn't seem to be involved in any pattern.

First you want a notation for repeating a string n times.
Let s be a string and define by induction, s^1 = s and s^(n+1) = ss^n.
Your set appears to be { x, y, yy, y^(4n) : n in N }.  
