A363236 Number of permutations p of [n] such that each element in p has at least one neighbor with opposite parity.

1, 0, 2, 2, 16, 36, 288, 1152, 10368, 57600, 604800, 4320000, 51840000, 453600000, 6147187200, 63605606400, 962415820800, 11500218777600, 192255565824000, 2605984690176000, 47721518530560000, 723526168780800000, 14407079038894080000, 241602987041095680000

Offset

0,3

Links

Alois P. Heinz, Table of n, a(n) for n = 0..465

Wikipedia, Permutation

Formula

a(n) ~ phi^n * n! / (5^(1/4) * 2^(n-1)), where phi = A001622 is the golden ratio. - Vaclav Kotesovec, May 26 2023

Example

a(0) = 1: (), the empty permutation.
a(1) = 0.
a(2) = 2: 12, 21.
a(3) = 2: 123, 321.
a(4) = 16: 1234, 1243, 1423, 1432, 2134, 2143, 2314, 2341, 3214, 3241, 3412, 3421, 4123, 4132, 4312, 4321.
a(5) = 36: 12345, 12354, 12534, 12543, 14325, 14352, 14523, 14532, 21345, 21543, 23145, 23541, 25143, 25341, 32145, 32154, 32514, 32541, 34125, 34152, 34512, 34521, 41325, 41523, 43125, 43521, 45123, 45321, 52134, 52143, 52314, 52341, 54123, 54132, 54312, 54321.

Crossrefs

Cf. A001622, A092186, A363180, A363181.

Sequence in context: A303567 A177832 A230800 * A076615 A098777 A127226

Adjacent sequences: A363233 A363234 A363235 * A363237 A363238 A363239

Keyword

nonn

Author

Alois P. Heinz, May 22 2023

Status

approved