OFFSET
1,3
COMMENTS
From Richard Guy's 1971 letter: "[Studied by] Helen Alderson, J. H. Conway, etc. at Cambridge. These are rooted trees with two branches at each stage and if A,B,C,D (see drawing [in letter]) are further growths, then one treats (AB)(CD) as equivalent to (AC)(BD) - otherwise one distinguishes left and right. [The sequence gives] the number of equivalence classes of such trees."
REFERENCES
R. K. Guy, personal communication.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
LINKS
Tyler Foster, A Noncommutative Version of the Natural Numbers, arXiv:1003.2081 [math.QA], 2010. See D(n) Table 2 p. 3.
R. K. Guy, Letter to N. J. A. Sloane, June 24 1971: front, back [Annotated scanned copy, with permission] See sequence C.
N. J. A. Sloane, Winter Fruits: New Problems from the OEIS, Dec. 2016 - Jan. 2017 (Part 1), Jan 26 2017.
Doron Zeilberger, Maple program for A002844
CROSSREFS
KEYWORD
nonn,more
AUTHOR
EXTENSIONS
Revised by N. J. A. Sloane, Dec 15 2016
a(11)-a(14) from Doron Zeilberger, Jan 31 2017
a(15)-a(16) from Sean A. Irvine, Sep 29 2023
STATUS
approved