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A001867 Number of n-bead necklaces with 3 colors.
(Formerly M2548 N1008)
22
1, 3, 6, 11, 24, 51, 130, 315, 834, 2195, 5934, 16107, 44368, 122643, 341802, 956635, 2690844, 7596483, 21524542, 61171659, 174342216, 498112275, 1426419858, 4093181691, 11767920118, 33891544419, 97764131646, 282429537947, 817028472960, 2366564736723 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

REFERENCES

J. Riordan, An Introduction to Combinatorial Analysis, Wiley, 1958, p. 162.

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

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

R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 7.112(a).

LINKS

T. D. Noe, Table of n, a(n) for n = 0..200

E. N. Gilbert and J. Riordan, Symmetry types of periodic sequences, Illinois J. Math., 5 (1961), 657-665.

INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 3

Juhani Karhumäki, S. Puzynina, M. Rao, M. A. Whiteland, On cardinalities of k-abelian equivalence classes, arXiv preprint arXiv:1605.03319 [math.CO], 2016.

J. Riordan, Letter to N. J. A. Sloane, Jul. 1978

Index entries for sequences related to necklaces

FORMULA

a(n) = (1/n)*Sum_{d|n} phi(d)*3^(n/d), n>0.

G.f.: 1 - Sum_{n>=1} phi(n)*log(1 - 3*x^n)/n). - Herbert Kociemba, Nov 01 2016

a(n) ~ 3^n/n. - Vaclav Kotesovec, Nov 01 2016

MAPLE

with(numtheory): A001867:= n-> `if` (n=0, 1, add (phi(d)* 3^(n/d), d=divisors(n))/n): seq (A001867(n), n=0..40);

spec := [N, {N=Cycle(bead), bead=Union(R, G, B), R=Atom, B=Atom, G=Atom}]; [seq(combstruct[count](spec, size=n), n=1..40)];

MATHEMATICA

Prepend[Table[CyclicGroupIndex[n, t]/.Table[t[i]->3, {i, 1, n}], {n, 1, 28}], 1]  (* Geoffrey Critzer, Sep 16 2011 *)

mx=40; CoefficientList[Series[1-Sum[EulerPhi[i] Log[1-3*x^i]/i, {i, 1, mx}], {x, 0, mx}], x] (* Herbert Kociemba, Nov 01 2016 *)

k=3; Prepend[Table[DivisorSum[n, EulerPhi[#] k^(n/#) &]/n, {n, 1, 30}], 1] (* Robert A. Russell, Sep 21 2018 *)

PROG

(PARI) a(n)=if (n==0, 1, 1/n * sumdiv(n, d, eulerphi(d)*3^(n/d) )); /* Joerg Arndt, Jul 04 2011 */

CROSSREFS

Column 3 of A075195.

Cf. A054610.

Sequence in context: A051284 A319910 A319636 * A000998 A221182 A109781

Adjacent sequences:  A001864 A001865 A001866 * A001868 A001869 A001870

KEYWORD

nonn,easy,nice,changed

AUTHOR

N. J. A. Sloane

STATUS

approved

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Last modified October 19 22:00 EDT 2018. Contains 316378 sequences. (Running on oeis4.)