Specifying periodic words by restrictions

Resumo

Consider a periodic sequence over a finite alphabet, say ..ababab.... This sequence can be specified by prohibiting the subwords aa and bb. In the paper, the maximum period of a word that can be defined by using k restrictions is determined. A sharp exponential bound is obtained: the period of a word determined by k restrictions cannot exceed the kth Fibonacci number. Thus, the period colength is estimated. The problem is studied in the context of Gröbner bases, namely, the growth of a Gröbner basis of an ideal (the cogrowth of an algebra). The proof uses the technique of Rauzy graphs.