

A173847


Number of permutations of 1..n with no adjacent pair summing to n+7.


0



1, 1, 2, 6, 24, 120, 720, 5040, 30240, 282240, 2338560, 26853120, 280143360, 3802982400, 47638886400, 745007155200, 10872058982400, 192275074252800, 3200021920972800, 63107717389516800, 1178953777845043200, 25641537378199142400, 531103116540719923200
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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 = 7; m = max(0,floor((nk+1)/2)); a(n,k) = Sum_{j=0..m} (2)^j C(m,j) (nj)!.


CROSSREFS

Sequence in context: A138113 A045977 A177278 * A154655 A256181 A293784
Adjacent sequences: A173844 A173845 A173846 * A173848 A173849 A173850


KEYWORD

nonn


AUTHOR

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


STATUS

approved



