2-layer straightline crossing minimization: Performance of exact and heuristic algorithms.
A k-abelian cube is a word uvw, where the factors u, v, and w are either pairwise equal, or have the same multiplicities for every one of their factors of length at most k. Previously it has been shown that k-abelian cubes are avoidable over a binary alphabet for k ≥ 8. Here it is proved that this holds for k ≥ 5.
We give analogs of the complexity and of Sturmian words which are called respectively the -complexity and -Sturmian words. We show that the class of -Sturmian words coincides with the class of words satisfying , and we determine the structure of -Sturmian words. For a class of words satisfying , we give a general formula and an upper bound for . Using this general formula, we give explicit formulae for for some words belonging to this class. In general, can take large values, namely,...