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 A108459 Number of labeled partitions of (n,n) into pairs (i,j). 22
 1, 1, 5, 52, 855, 19921, 614866, 24040451, 1152972925, 66200911138, 4465023867757, 348383154017581, 31052765897026352, 3128792250765898965, 353179564583216567917, 44320731930172534543092, 6141797839043095806714667, 934330605640859569909566925 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Partitions of n black objects labeled 1..n and n white objects labeled 1..n. Each partition must have at least one white object. a(n) is also the number of elements of the partition monoid P_n with domain {1,...,n}. Elements of P_n are set partitions of {1,1',...,n,n'}, and the domain of such a partition is the set of all points in {1,...,n} that belong to a block containing a dashed element. - James East, Apr 10 2018 LINKS Seiichi Manyama, Table of n, a(n) for n = 0..200 FORMULA a(n) = Sum_{k=0..n} k^n*Stirling2(n,k). - Vladeta Jovovic, Aug 31 2006 E.g.f.: Sum_{n>=0} (exp(n*x)-1)^n / n!. - Vladeta Jovovic, Jul 12 2007 E.g.f.: Sum_{n>=0} exp(n^2*x) * exp( -exp(n*x) ) / n!. - Paul D. Hanna, Jun 28 2019 O.g.f.: Sum_{n>=0} n^n * x^n / Product_{k=1..n} (1 - n*k*x). - Paul D. Hanna, Sep 17 2013 a(n) = Sum_{k=0..n} Stirling2(n,k) * Sum_{l=k..n} Stirling2(n,l)*T(l,k). Here T(l,k) are the falling factorials. - James East, Apr 10 2018 MAPLE b:= proc(n) option remember; expand(`if`(n=0, 1, x*add(b(n-j)*binomial(n-1, j-1), j=1..n))) end: a:= n-> add(coeff(b(n), x, j)*j^n, j=0..n): seq(a(n), n=0..21); # Alois P. Heinz, Dec 02 2023 PROG (PARI) {a(n)=polcoeff(sum(m=0, n, m^m*x^m/prod(k=1, m, 1-m*k*x +x*O(x^n))), n)} \\ Paul D. Hanna, Sep 17 2013 (PARI) {a(n)=n!*polcoeff(sum(m=0, n, (exp(m*x+x*O(x^n))-1)^m/m!), n)} \\ Paul D. Hanna, Sep 17 2013 CROSSREFS Main diagonal of A108458. Cf. A108461. Cf. A048993 (Stirling2), A068424 (falling factorial). Cf. A326600, A326270, A326271, A326288. Bisection of A124421 (even part). Sequence in context: A367165 A357346 A196531 * A223898 A210096 A076281 Adjacent sequences: A108456 A108457 A108458 * A108460 A108461 A108462 KEYWORD nonn AUTHOR Christian G. Bower, Jun 03 2005 STATUS approved

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Last modified July 12 22:00 EDT 2024. Contains 374257 sequences. (Running on oeis4.)