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A084423 Set partitions up to rotations. 10
1, 1, 2, 3, 7, 12, 43, 127, 544, 2361, 11703, 61690, 351773, 2126497, 13639372, 92197523, 655035769, 4874404108, 37893370473, 306986431847, 2586209749712, 22612848403571, 204850732480285, 1919652428481930, 18581619724363401, 185543613289200949 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

Partitions of n objects distinct under the cyclic group, C_n. By comparison the partition numbers (A000041) are the partitions distinct under the symmetric group, S_n and the set partitions are those distinct under the discrete group containing only the identity. - Franklin T. Adams-Watters, Jun 09 2008

Equivalently, number of n-bead necklaces using any number of unlabeled (interchangable) colors. - Andrew Howroyd, Sep 25 2017

LINKS

Franklin T. Adams-Watters and Alois P. Heinz, Table of n, a(n) for n = 0..500 (first 61 terms from Franklin T. Adams-Watters)

Robert M. Dickau, Bell number diagrams

Wouter Meeussen, Set Partitions Up To Rotation

Tilman Piesk, Partition related number triangles

FORMULA

a(p) = (Bell(p)+2*(p-1))/p for prime p; cf. A079609. - Vladeta Jovovic, Jul 04 2003

U(k,j) = 1 if k=0, else Sum_{i=1..k} C(k-1,i-1) Sum_{d|j} U(k-i,j)*d^{i-1}. Then a(n) = (Sum_{j|n} phi(j)*U(n/j,j))/n. (U(k,j) is the number of partitions invariant under a permutation with k cycles of j objects each.) - Franklin T. Adams-Watters, Jun 09 2008

a(n) = [n==0] + [n>0] * (1/n) * Sum_{d|n} phi(d) * A162663(n/d,d). - Robert A. Russell, Jun 10 2018

EXAMPLE

Of the Bell(4) = 15 set partitions of 4, only 7 remain distinct under rotation:

{{1,2,3,4}},

{{1}, {2,3,4}},

{{1,2}, {3,4}},

{{1,3}, {2,4}},

{{1}, {2}, {3,4}},

{{1}, {3}, {2,4}},

{{1}, {2}, {3}, {4}}}

MATHEMATICA

<<DiscreteMath`Combinatorica`; shrink[n_Integer] := Union[ First[ Sort[ NestList[Sort[Sort /@ ( #/.i_Integer:>Mod[i+1, n, 1])]&, #, n]]]& /@ SetPartitions[n]]; Table[ Length[ shrink[k]], {k, 11}]

u[0, _] = 1; u[k_, j_] := u[k, j] = Sum[Binomial[k-1, i-1]*Sum[u[k-i, j]*d^(i-1), {d, Divisors[j]}], {i, 1, k}]; a[n_] := Sum[EulerPhi[j]*u[n/j, j], {j, Divisors[n]}]/n; a[0] = 1; Table[a[n], {n, 0, 24}] (* Jean-Fran├žois Alcover, May 14 2012, after Franklin T. Adams-Watters *)

PROG

(PARI) U(k, j) = if(k==0, 1, sum(i=1, k, binomial(k-1, i-1)*sumdiv(j, d, U(k-i, j) *d^(i-1)))) /* U is unoptimized; should remember previous values. */

a(n) = sumdiv(n, j, eulerphi(j)*U(n\j, j))/n \\ Franklin T. Adams-Watters, Jun 09 2008

CROSSREFS

Cf. A080107, A084708, A152175.

Sequence in context: A056293 A275311 A056294 * A068134 A249051 A225093

Adjacent sequences:  A084420 A084421 A084422 * A084424 A084425 A084426

KEYWORD

nonn,nice

AUTHOR

Wouter Meeussen, Jun 26 2003

EXTENSIONS

More terms from Robert G. Wilson v, Jun 27 2003

More terms from Franklin T. Adams-Watters, Jun 09 2008

STATUS

approved

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Last modified October 16 08:24 EDT 2018. Contains 316259 sequences. (Running on oeis4.)