A190825 Number of permutations of 2 copies of 1..n introduced in order 1..n with no element equal to another within a distance of 4.

1, 0, 0, 0, 0, 1, 40, 876, 16924, 332507, 6944594, 156127796, 3783620060, 98614428186, 2754950165519, 82200735013648, 2610496155216456, 87952061484214504, 3134296573781405311, 117816169705165137052, 4659486136831587519216, 193431371632142599974128

Offset: 0

Author

R. H. Hardin, May 21 2011

Keywords

• nonn

Examples

			Some solutions for n=6
..1....1....1....1....1....1....1....1....1....1....1....1....1....1....1....1
..2....2....2....2....2....2....2....2....2....2....2....2....2....2....2....2
..3....3....3....3....3....3....3....3....3....3....3....3....3....3....3....3
..4....4....4....4....4....4....4....4....4....4....4....4....4....4....4....4
..5....5....5....5....5....5....5....5....5....5....5....5....5....5....5....5
..6....6....6....6....6....6....6....6....6....6....1....1....1....6....6....1
..1....2....1....2....1....1....2....2....1....2....6....6....6....1....2....6
..2....3....3....3....3....2....3....3....3....3....3....2....2....3....1....2
..3....1....2....1....4....3....1....4....2....4....4....4....3....4....4....4
..5....4....5....5....5....4....5....5....4....1....2....3....5....5....5....5
..4....6....6....4....2....6....6....1....6....5....5....5....4....6....3....3
..6....5....4....6....6....5....4....6....5....6....6....6....6....2....6....6
		

Links

• Alois P. Heinz, Table of n, a(n) for n = 0..404
• Everett Sullivan, Linear chord diagrams with long chords, arXiv preprint arXiv:1611.02771 [math.CO], 2016.

Crossrefs

Column k=5 of A293157.

Formula

a(n) ~ 2^(n + 1/2) * n^n / exp(n+4). - Vaclav Kotesovec, Oct 26 2017

Extensions

a(0), a(18)-a(21) from Alois P. Heinz, Oct 17 2017