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A007056 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)
4
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

REFERENCES

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

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.

FORMULA

a(n) = A187273(n) - Sum_{d|n,d<n} phi(n/d)*a(d). - Sean A. Irvine, Sep 27 2017

MAPLE

See A007055.

CROSSREFS

Column 3 of A284873.

Sequence in context: A005254 A272265 A191796 * A026551 A296719 A060578

Adjacent sequences:  A007053 A007054 A007055 * A007057 A007058 A007059

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Mira Bernstein, R. Kemp

EXTENSIONS

Entry revised by N. J. A. Sloane, Mar 07 2011

STATUS

approved

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Last modified January 22 15:57 EST 2019. Contains 319364 sequences. (Running on oeis4.)