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A181142 Number of permutations of {1,2,...,2n} , say x(1),x(2), ... , x(2n) , such that x(i) + x(i+1) is not equal to 2n-1 for all i, 1<=i<=2n-1. 0
2, 12, 336, 17760, 1543680, 199019520, 35611269120, 8437755432960, 2556188496691200, 963558923688345600, 442230750973683302400, 242766600433441859174400, 157060798435284559803187200 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..13.

Nathan Reading, Assignments

StackExchange, Can this be simplified?

FORMULA

a(n) = \sum_{k=0}^{n-1} { n -1 \choose k } (-2)^{k} (2n - k)!

MATHEMATICA

f[n_] := Sum[(-2)^k (2 n - k)! Binomial[n - 1, k], {k, 0, n - 1}]; Array[f, 13] (* Robert G. Wilson v, Oct 16 2010 *)

PROG

(Other) SAS datastep: data _null_; do n = 1 to 7; a = 0; do _n_ = 0 to n-1; a = a + (-2)**_n_ * comb(n-1, _n_)*fact(2*n-_n_); end; output; put "a(" n ")=" a; end; run;

CROSSREFS

Sequence in context: A012727 A296622 A325756 * A088229 A060596 A074257

Adjacent sequences:  A181139 A181140 A181141 * A181143 A181144 A181145

KEYWORD

nonn

AUTHOR

Arin Chaudhuri (arin.chaudhuri(AT)gmail.com), Oct 06 2010

EXTENSIONS

a(8) and onward from Robert G. Wilson v, Oct 16 2010

STATUS

approved

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Last modified January 18 01:05 EST 2020. Contains 330995 sequences. (Running on oeis4.)