

A289674


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


9



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

1,1


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.


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. 8399. [Annotated scanned copy]


EXAMPLE

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).


CROSSREFS

Cf. A284116, A284119, A284121, A289670A289675.
Sequence in context: A109029 A284915 A252165 * A217265 A236644 A178282
Adjacent sequences: A289671 A289672 A289673 * A289675 A289676 A289677


KEYWORD

nonn


AUTHOR

N. J. A. Sloane, Jul 29 2017


EXTENSIONS

Corrected and extended by Don Reble, Jul 31 2017
Terms sorted and more terms added by Chai Wah Wu, Aug 05 2017


STATUS

approved



