When considering universal cycles for a specific set S, there are several important questions: Does a universal cycle exist
for S? What is the number of universal cycles for S? How can a specific universal cycle for S be constructed? Is there an
efficient algorithm that constructs a universal cycle for S? The last two questions can also be put for the lexicographically
smallest universal cycle for S. By lexicographically smallest, we mean that the linear representation is the smallest possible in
lexicographic order. For instance, the universal cycle from our example is the lexicographically smallest for B(4). (The term
minimal is also used in the literature [19,20] for the same concept.)
The lexicographically smallest universal cycle for B(n) was first constructed by Martin in the 1930s [18]. The author
showed that the lexicographically smallest universal cycle for B(n) can be constructed by a greedy algorithm that uses
exponential space. Later, Fredricksen, Kessler and Maiorana provided a more direct method in [8] for constructing this