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A007057 Let S denote the palindromes in the language {0,1,2,3}*; a(n) = number of words of length n in the language SS.
(Formerly M3510)
4
1, 4, 16, 40, 136, 304, 880, 1768, 4936, 9112, 25216, 45016, 121600, 212944, 571552, 982240, 2616136, 4456384, 11785408, 19922872, 52402336, 88076560, 230641504, 385875880, 1006499200, 1677720304, 4361862976, 7247738776, 18789905872, 31138512784, 80529599680, 133143986056, 343594756936 (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..32.

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) = A187274(n) - Sum_{d|n,d<n} phi(n/d)*a(d). - Sean A. Irvine, Sep 27 2017

MAPLE

See A007055.

CROSSREFS

Column 4 of A284873.

Sequence in context: A210440 A220499 A110477 * A206918 A056373 A018828

Adjacent sequences:  A007054 A007055 A007056 * A007058 A007059 A007060

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 October 22 22:34 EDT 2019. Contains 328335 sequences. (Running on oeis4.)