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A056310
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Number of reversible strings with n beads using exactly three different colors.
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2
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0, 0, 3, 18, 78, 273, 921, 2916, 9150, 28065, 85773, 259848, 785778, 2367813, 7128201, 21427956, 64382550, 193326105, 580372293, 1741847328, 5227116378, 15684323853, 47059266081, 141189861996
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OFFSET
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1,3
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COMMENTS
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A string and its reverse are considered to be equivalent.
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REFERENCES
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M. R. Nester (1999). Mathematical investigations of some plant interaction designs. PhD Thesis. University of Queensland, Brisbane, Australia.
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LINKS
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Table of n, a(n) for n=1..24.
Index to sequences with linear recurrences with constant coefficients, signature (6,-6,-24,49,6,-66,36).
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FORMULA
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A032120(n)-3*A005418(n+1)+3.
G.f.: -3*x^3*(12*x^4-5*x^3-4*x^2+1)/((x-1)*(2*x-1)*(3*x-1)*(2*x^2-1)*(3*x^2-1)). [Colin Barker, Jul 07 2012]
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CROSSREFS
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Cf. A032120.
Sequence in context: A094033 A043008 A056319 * A135371 A086346 A036290
Adjacent sequences: A056307 A056308 A056309 * A056311 A056312 A056313
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KEYWORD
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nonn,easy
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AUTHOR
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Marks R. Nester (nesterm(AT)dpi.qld.gov.au)
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STATUS
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approved
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