A177283 Number of permutations of 2 copies of 1..n with all adjacent differences <= 2 in absolute value.

1, 1, 6, 90, 660, 3846, 24326, 148146, 850920, 4788114, 26548704, 144707622, 778116870, 4140736398, 21836757474, 114262071558, 593931441790, 3069689137294, 15786994900896, 80840741811238, 412406022128206, 2096937750144962, 10631278302356284, 53761972675284874

Offset: 0

Author

R. H. Hardin, May 06 2010

Keywords

• nonn

Links

• Andrew Howroyd, Table of n, a(n) for n = 0..200

Crossrefs

Cf. A000680.

Formula

a(n) = (2n)!/2^n for n<=3.

Extensions

Terms a(16) and beyond from Andrew Howroyd, May 14 2020