

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 2n1 for all i, 1<=i<=2n1.


0



2, 12, 336, 17760, 1543680, 199019520, 35611269120, 8437755432960, 2556188496691200, 963558923688345600, 442230750973683302400, 242766600433441859174400, 157060798435284559803187200
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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}^{n1} { 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 n1; a = a + (2)**_n_ * comb(n1, _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



