A289674 Consider the Post tag system described in A284116 (but adapted to the alphabet {1,2}); sequence lists the words that belong to cycles.

21211, 112212, 21211221211, 112212112212, 221222121111, 1112212221211, 2122212111111, 2221211112212, 12111122122212, 12221222121111, 22121111112212, 122122212221211, 211111122122212, 1111221222122212, 21211221211221211, 21222122212111111, 112212112212112212

Offset: 1

Author

N. J. A. Sloane, Jul 29 2017

Keywords

• nonn

Comments

Post's tag system maps a word w over {1,2} to w', where if w begins with 1, w' is obtained by appending 11 to w and deleting the first three letters, or if w begins with 2, w' is obtained by appending 2212 to w and deleting the first three letters.

Under this Post tag system, some words when iterated end at the empty word, others go into cycles, and others may have an orbit which grows without limit. See A289670 and A289671 for the counts of the first two types. This sequence gives a list of the words that belong to cycles.

It is an important open question to decide if there is any word whose orbit grows without limit.

We work over {1,2} rather than the official alphabet {0,1} because of the prohibition in the OEIS of terms (other than 0 itself) which begin with 0.

Examples

			The first two cycles that one encounters when applying the Post tag system to words over the alphabet {1,2} are (21211, 112212) and (2122212111111, 22121111112212, 211111122122212, 1111221222122212, 122122212221211, 12221222121111).
		

Links

• Chai Wah Wu, Table of n, a(n) for n = 1..253 (terms < 10^49)
• Shigeru Watanabe, Periodicity of Post's normal process of tag, in Jerome Fox, ed., Proceedings of Symposium on Mathematical Theory of Automata, New York, April 1962, Polytechnic Press, Polytechnic Institute of Brooklyn, 1963, pp. 83-99. [Annotated scanned copy]

Crossrefs

Cf. A284116, A284119, A284121, A289670-A289675.

Extensions

Corrected and extended by Don Reble, Jul 31 2017

Terms sorted and more terms added by Chai Wah Wu, Aug 05 2017