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.
FORMULA
k = 7; m = max(0,floor((n-k+1)/2)); a(n,k) = Sum_{j=0..m} (-2)^j C(m,j) (n-j)!.
CROSSREFS
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