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

0,7

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.

Formula

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

Example

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

Crossrefs

Column k=5 of A293157.

Sequence in context: A004421 A247411 A250986 * A054340 A271030 A013350

Adjacent sequences: A190822 A190823 A190824 * A190826 A190827 A190828

Keyword

nonn

Author

R. H. Hardin, May 21 2011

Extensions

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

Status

approved