Let Double be the set of strings defined as {s} s= ww}. Double contains only strings composed of two identical repeated pieces. For example, if you have a vocabulary of the ten digits 0 to 9, then the following strings (and many more!) are in Double: 11, 1212, 123123, 767767, 98769876, . . . .
Assume you have a vocabulary consisting only of the single letter a. Is Double a regular set? Why?
Assume you now have a vocabulary consisting of the two letters, a and b. Is Double a regular set? Why?