

A173843


Number of permutations of 1..n with no adjacent pair summing to n+3.


0



1, 1, 2, 6, 12, 72, 336, 2640, 17760, 175680, 1543680, 18385920, 199019520, 2771112960, 35611269120, 567422392320, 8437755432960, 151385755852800, 2556188496691200, 50989753530777600, 963558923688345600, 21152552961009254400, 442230750973683302400
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OFFSET

0,3


COMMENTS

If a(n,k) is the number of permutations of 1..n with no adjacent pair summing to n+k, then a(n,k)=a(n,k+1) for n+k even.


LINKS

Table of n, a(n) for n=0..22.


FORMULA

k = 3; m = \max (0,floor((nk+1)/2)); a(n,k) = \sum_{j=0}^m (2)^j \binom{m}{j} (nj)!


CROSSREFS

Sequence in context: A076220 A258213 A178846 * A107763 A166470 A144144
Adjacent sequences: A173840 A173841 A173842 * A173844 A173845 A173846


KEYWORD

nonn


AUTHOR

R. H. Hardin Feb 26 2010, comment proved by William Keith, formula from Max Alekseyev, on the Sequence Fans Mailing List


EXTENSIONS

More terms from Alois P. Heinz, Jan 09 2017


STATUS

approved



