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A323870 Number of toroidal necklaces of size n whose entries cover an initial interval of positive integers. 13
1, 4, 10, 61, 218, 3136, 13514, 272998, 2362439, 40899248, 295024106, 14045787790, 81055130522, 3040383719360, 61408850927732, 1661142088494553, 15337737297545402, 1128511554421317128, 9768588138876674858, 803306338873366385030, 15452347618762680757428 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

We define a toroidal necklace to be an equivalence class of matrices under all possible rotations of the sequence of rows and the sequence of columns. Alternatively, a toroidal necklace is a matrix that is minimal among all possible rotations of its sequence of rows and its sequence of columns.

LINKS

Andrew Howroyd, Table of n, a(n) for n = 1..200

S. N. Ethier, Counting toroidal binary arrays, J. Int. Seq. 16 (2013) #13.4.7.

EXAMPLE

The a(3) = 10 toroidal necklaces:

  [1 2 3] [1 3 2] [1 2 2] [1 1 2] [1 1 1]

.

  [1] [1] [1] [1] [1]

  [2] [3] [2] [1] [1]

  [3] [2] [2] [2] [1]

MATHEMATICA

sps[{}]:={{}}; sps[set:{i_, ___}]:=Join@@Function[s, Prepend[#, s]&/@sps[Complement[set, s]]]/@Cases[Subsets[set], {i, ___}];

nrmmats[n_]:=Join@@Table[Table[Table[Position[stn, {i, j}][[1, 1]], {i, d}, {j, n/d}], {stn, Join@@Permutations/@sps[Tuples[{Range[d], Range[n/d]}]]}], {d, Divisors[n]}];

neckmatQ[m_]:=m==First[Union@@Table[RotateLeft[m, {i, j}], {i, Length[m]}, {j, Length[First[m]]}]];

Table[Length[Select[nrmmats[n], neckmatQ]], {n, 6}]

PROG

(PARI)

U(n, m, k) = (1/(n*m)) * sumdiv(n, c, sumdiv(m, d, eulerphi(c) * eulerphi(d) * k^(n*m/lcm(c, d))));

R(v)={sum(n=1, #v, sum(k=1, n, (-1)^(n-k)*binomial(n, k)*v[k]))}

a(n)={if(n < 1, n==0, R(vector(n, k, sumdiv(n, d, U(d, n/d, k))) ))} \\ Andrew Howroyd, Aug 18 2019

CROSSREFS

Cf. A000670, A008965, A060223.

Cf. A323858, A323859, A323866, A323868, A323871.

Sequence in context: A209030 A220824 A124724 * A298162 A203226 A300642

Adjacent sequences:  A323867 A323868 A323869 * A323871 A323872 A323873

KEYWORD

nonn

AUTHOR

Gus Wiseman, Feb 04 2019

EXTENSIONS

Terms a(9) and beyond from Andrew Howroyd, Aug 18 2019

STATUS

approved

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Last modified October 16 03:52 EDT 2021. Contains 348035 sequences. (Running on oeis4.)