A180890 Number of permutations of 2..n+1 with no element greater than or equal to the sum of its neighbors

1, 0, 2, 6, 20, 90, 550, 3512, 25378, 200558, 1778156, 17016488, 177741892, 1984308356, 23873823014, 304944376412, 4155112842840

Offset

1,3

Links

Table of n, a(n) for n=1..17.

Example

All solutions for n=5
(2 3 4 6 5) (2 3 5 6 4) (2 3 6 5 4) (2 4 3 6 5)
(2 4 5 6 3) (2 4 6 5 3) (2 5 4 6 3) (2 5 6 4 3)
(2 6 5 4 3) (3 4 2 6 5) (3 4 5 6 2) (3 4 6 5 2)
(3 5 6 4 2) (3 6 4 5 2) (3 6 5 4 2) (4 5 6 3 2)
(4 6 5 3 2) (5 6 2 4 3) (5 6 3 4 2) (5 6 4 3 2)

Crossrefs

Sequence in context: A318813 A336544 A000666 * A027321 A027315 A005965

Adjacent sequences: A180887 A180888 A180889 * A180891 A180892 A180893

Keyword

nonn

Author

R. H. Hardin Sep 23 2010

Status

approved