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A007056
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Let S denote the palindromes in the language {0,1,2}*; a(n) = number of words of length n in the language SS.
(Formerly M2781)
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4
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1, 3, 9, 21, 57, 123, 279, 549, 1209, 2127, 4689, 7989, 17031, 28395, 60615, 98061, 208569, 334563, 705789, 1121877, 2356737, 3718827, 7786359, 12223077, 25488903, 39857523, 82876257, 129135729, 267784119, 416118219, 860825439, 1334448261, 2754778809, 4261609131, 8781196329, 13559714109, 27893530029
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OFFSET
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0,2
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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Table of n, a(n) for n=0..36.
R. Kemp, On the number of words in the language {w in Sigma* | w = w^R }^2, Discrete Math., 40 (1982), 225-234.
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FORMULA
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a(n) = A187273(n) - Sum_{d|n,d<n} phi(n/d)*a(d). - Sean A. Irvine, Sep 27 2017
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MAPLE
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See A007055.
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CROSSREFS
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Column 3 of A284873.
Sequence in context: A005254 A272265 A191796 * A026551 A296719 A060578
Adjacent sequences: A007053 A007054 A007055 * A007057 A007058 A007059
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KEYWORD
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nonn
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AUTHOR
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N. J. A. Sloane, Mira Bernstein, R. Kemp
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EXTENSIONS
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Entry revised by N. J. A. Sloane, Mar 07 2011
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STATUS
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approved
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