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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. [proved by William Keith] LINKS FORMULA k = 3; a(n,k) = Sum_{j=0..m} (-2)^j*binomial(m,j)*(n-j)! where m = max(0, floor((n-k+1)/2)). [Max Alekseyev, on the Sequence Fans Mailing List] 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 EXTENSIONS More terms from Alois P. Heinz, Jan 09 2017 STATUS approved

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Last modified January 28 06:00 EST 2021. Contains 340490 sequences. (Running on oeis4.)