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A092297 Number of ways of 3-coloring an annulus consisting of n zones joined like a pearl necklace. 5
0, 6, 6, 18, 30, 66, 126, 258, 510, 1026, 2046, 4098, 8190, 16386, 32766, 65538, 131070, 262146, 524286, 1048578, 2097150, 4194306, 8388606, 16777218, 33554430, 67108866, 134217726, 268435458, 536870910, 1073741826, 2147483646 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

A circular domain means a domain between two concentric circles and it is divided into n parts by n boundary lines perpendicular to the circles. Both sides of a line must have different colors. How many ways of coloring are there?

a(n) is also the multiple of six that's nearest to 2^n. [David Eppstein, Aug 31 2010]

a(n) apparently is the trace of the n-th power of the adjacency matrix of the complete 3-graph, a 3x3 matrix with diagonal elements all zero and off-diagonal all ones (cf. A001045). If so, a(n) is the number of closed walks on the graph of length n. - Tom Copeland, Nov 06 2012

For n>=2, a(n) is the number of length n words on 3 letters with no two consecutive like letters including the first and the last.  Cf. A218034. - Geoffrey Critzer, Apr 05 2014

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Index to sequences with linear recurrences with constant coefficients, signature (1,2).

FORMULA

a(n) = 2^n+2*(-1)^n; recurrence a(1)=0, a(2)=6, a(n) = 2*a(n-2)+a(n-1).

O.g.f: -6*x^2/((1+x)*(2*x-1)) = -3-1/(2*x-1)+2/(1+x). - R. J. Mathar, Dec 02 2007

a(n) = 6*A001045(n-1). - R. J. Mathar, Aug 30 2008

EXAMPLE

a(2)=6 because we can color one zone in 3 colors and the other in 2, so 2*3=6 in all.

MATHEMATICA

nn=28; Drop[CoefficientList[Series[6x^2/(1+x)^2/(1-3x/(1+x)), {x, 0, nn}], x], 1] (* Geoffrey Critzer, Apr 05 2014 *)

PROG

(MAGMA) [2^n+2*(-1)^n : n in [1..40]]; // Vincenzo Librandi, Sep 27 2011

CROSSREFS

Sequence in context: A151724 A000976 A161787 * A224711 A073096 A212622

Adjacent sequences:  A092294 A092295 A092296 * A092298 A092299 A092300

KEYWORD

nonn,easy

AUTHOR

S. B. Step (stepy(AT)vesta.ocn.ne.jp), Feb 06 2004

STATUS

approved

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Last modified April 24 03:15 EDT 2014. Contains 240947 sequences.