tailieunhanh - Báo cáo toán học: " Binary words containing infinitely many overlaps"

Tuyển tập các báo cáo nghiên cứu khoa học về toán học trên tạp chí toán học quốc tế đề tài: Binary words containing infinitely many overlaps. | Binary words containing infinitely many overlaps James Currie Department of Mathematics University of Winnipeg Winnipeg Manitoba R3B 2E9 Canada Narad Rampersad Jeffrey Shallit School of Computer Science University of Waterloo Waterloo Ontario N2L 3G1 Canada nrampersad@ shallit@ Submitted Nov 16 2005 Accepted Sep 15 2006 Published Sep 22 2006 Mathematics Subject Classifications 68R15 Abstract We characterize the squares occurring in infinite overlap-free binary words and construct various a power-free binary words containing infinitely many overlaps. 1 Introduction If a is a rational number a word w is an a power if there exists words x and x with x a prefix of x such that w xnx and a n x x . We refer to x as a period of w. An a power is a word that is a d power for some d a. A word is a power-free resp. a power-free if none of its subwords is an a power resp. a power . A 2 power is called a square a 2 power is called an overlap. Thue 18 constructed an infinite overlap-free binary word however Dekking 8 showed that any such infinite word must contain arbitrarily large squares. Shelton and Soni 17 characterized the overlap-free squares but it is not hard to show that there are some overlap-free squares such as 00110011 that cannot occur in an infinite overlap-free binary word. In this paper we characterize those overlap-free squares that do occur in infinite overlap-free binary words. Shur 16 considered the bi-infinite overlap-free and 7 3 power-free binary words and showed that these classes of words were identical. There have been several subsequent papers 1 10 11 14 that have shown various similarities between the classes of overlap-free binary words and 7 3 power-free binary words. Here we contrast the two classes of words THE ELECTRONIC JOURNAL OF COMBINATORICS 13 2006 R82 1 by showing that there exist one-sided infinite 7 3 power-free binary words containing infinitely many overlaps. More .

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