A177285 Number of permutations of 2 copies of 1..n with all adjacent differences <= 4 in absolute value.

1, 1, 6, 90, 2520, 113400, 3356640, 75336660, 1500131820, 29660400822, 620330067782, 13125403608162, 274194614811528, 5605288189594200, 112437639146871360, 2235073249396108440, 44209749453949113840, 870798759948756386976, 17069437222534487122608, 332789644153015982087550, 6455422726721469064982952

Offset: 0

Author

R. H. Hardin, May 06 2010

Keywords

• nonn

Crossrefs

Cf. A177283.

Formula

a(n) = (2n)!/2^n for n<=5.

Extensions

a(0)=1 prepended and terms a(13) and beyond from Andrew Howroyd, May 14 2020