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 A116218 If X_1,...,X_n is a partition of a 2n-set X into 2-blocks (or pairs) then a(n) is equal to the number of permutations f of X such that f(X_i) != X_i for all i=1,...n. 4
 0, 20, 592, 35088, 3252608, 437765440, 80766186240, 19580003614976, 6038002429456384, 2308538525796209664, 1071858241055770480640, 594103565746026102722560, 387504996819754568329494528, 293818792387460667662661926912, 256273357771747968541309427187712 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS G. C. Greubel, Table of n, a(n) for n = 1..220 Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets FORMULA a(n) = Sum_{i=0,..,n} (-2)^i*binomial(n,i)*(2*n-2*i)!. Recurrence: a(n) = 2*(n-1)*(2*n+1)*a(n-1) + 4*(n-1)*(4*n-3)*a(n-2) + 16*(n-2)*(n-1)*a(n-3). - Vaclav Kotesovec, Mar 20 2014 a(n) ~ sqrt(Pi) * 2^(2*n+1) * n^(2*n+1/2) / exp(2*n). - Vaclav Kotesovec, Mar 20 2014 EXAMPLE a(5)=3252608 MAPLE a:=n->sum((-2)^i*binomial(n, i)*(2*n-2*i)!, i=0..n); MATHEMATICA Table[Sum[(-2)^i*Binomial[n, i]*(2*n-2*i)!, {i, 0, n}], {n, 1, 20}] (* Vaclav Kotesovec, Mar 20 2014 *) PROG (PARI) for(n=1, 25, print1(sum(i=0, n, (-2)^i*binomial(n, i)*(2*n-2*i)!), ", ")) \\ G. C. Greubel, Mar 18 2017 CROSSREFS Sequence in context: A259057 A184123 A027407 * A035279 A015268 A202577 Adjacent sequences:  A116215 A116216 A116217 * A116219 A116220 A116221 KEYWORD nonn AUTHOR Milan Janjic, Apr 08 2007, corrected Apr 13 2007 STATUS approved

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Last modified August 3 17:04 EDT 2021. Contains 346439 sequences. (Running on oeis4.)