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A023997 Number of block permutations on an n-set. 8
1, 1, 3, 25, 339, 6721, 179643, 6166105, 262308819, 13471274401, 818288740923, 57836113793305, 4693153430067699, 432360767273547841, 44794795522199781243, 5176959027946049635225, 662704551840482536170579 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

A block permutation of a set X is a bijection between two quotient sets of X (of necessarily equal rank).

Number of labeled partitions of (n,n) into pairs (i,j) where there are n black objects labeled 1..n and n white objects labeled 1..n. Each partition must have at least one black object and at least one white object. - Christian G. Bower, Jun 03 2005

LINKS

Vaclav Kotesovec, Table of n, a(n) for n = 0..288 (terms 0..45 from Vincenzo Librandi)

D. G. FitzGerald and Jonathan Leech, Dual symmetric inverse monoids and representation theory, J. Australian Mathematical Society (Series A), Vol. 64 (1998), pp. 345-367.

FORMULA

a(0)=1, a(n) = Sum_{k=1..n} k! * S2(n,k)^2, S2(n,k) are the Stirling numbers of the second kind.

EXAMPLE

For n=3, there are the 3! ordinary permutations (of rank 3), 18 block permutations of rank 2 (2! for each pair of partitions of rank 2) and the single rank 1 one.

MATHEMATICA

Table[Sum[StirlingS2[n, k]^2k!, {k, 0, n}], {n, 0, 100}] (* Emanuele Munarini, Jul 04 2011 *)

PROG

(Maxima) makelist(sum(stirling2(n, k)^2*k!, k, 0, n), n, 0, 24); /* Emanuele Munarini, Jul 04 2011 */

(PARI) a(n) = if (n==0, 1, sum(k=1, n, k!*stirling(n, k, 2)^2)); \\ Michel Marcus, Jun 18 2019

CROSSREFS

Cf. A023998, A002720, A014235, A111420.

Sequence in context: A181085 A143635 A246756 * A154961 A322760 A325286

Adjacent sequences:  A023994 A023995 A023996 * A023998 A023999 A024000

KEYWORD

easy,nonn,nice

AUTHOR

Des FitzGerald (D.FitzGerald(AT)utas.edu.au)

EXTENSIONS

More terms from Christian G. Bower, Jun 03 2005

STATUS

approved

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Last modified June 13 11:17 EDT 2021. Contains 344990 sequences. (Running on oeis4.)