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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 (list; graph; refs; listen; history; text; internal format)
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((n-k+1)/2)); a(n,k) = \sum_{j=0}^m (-2)^j \binom{m}{j} (n-j)!

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

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Last modified June 27 11:25 EDT 2017. Contains 288788 sequences.