Determining Sets of Quasiperiods of Infinite Words

A word is quasiperiodic if it can be obtained by concatenations and overlaps of a smaller word, called a quasiperiod. Based on links between quasiperiods, right special factors and square factors, we introduce a method to determine the set of quasiperiods of a given right infinite word. Then we study the structure of the sets of quasiperiods of right infinite words and, using our method, we provide examples of right infinite words with extremal sets of quasiperiods (no quasiperiod is quasiperiodic, all quasiperiods except one are quasiperiodic, ...). Our method is also used to provide a short proof of a recent characterization of quasiperiods of the Fibonacci word. Finally we extend this result to a new characterization of standard Sturmian words using a property of their sets of quasiperiods

Quasiperiodic Sturmian words and morphisms

AbstractWe characterize all quasiperiodic Sturmian words: A Sturmian word is not quasiperiodic if and only if it is a Lyndon word. Moreover, we study links between Sturmian morphisms and quasiperiodicity

Some Properties of Quasiperiodic Subshifts

Let be a finite alphabet. A word w over is said to be quasiperiodic if it has a finite proper subword q such that every position of w falls under some occurrence of q. In this case, q is said to be a quasiperiod of w. A quasiperiodic word with an infinite number of quasiperiods is called multi-scale quasiperiodic. We show that, unlike in the case of right infinite words, biinfinite multi-scale quasiperiodicity does not imply uniform recurrence. The relation between biinfinite quasiperiodicity and other notions of symmetry of words were explored. It was also shown that the q-quasiperiodic subshift Xq, which is the set of all q-quasiperiodic biinfinite words, is a (2|q| – 2)-memory shift of finite type. By identifying all periodic points in Xq, a necessary and sufficient condition for the q-quasiperiodic subshift Xq to be mixing was established. Lastly, disjoint unions of these qi-quasiperiodic subshifts, where the qi’s are non-empty finite words over , were looked into. A sufficient condition for such a union be a shift of finite type was given