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 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. 2
 1, 0, 0, 0, 0, 1, 40, 876, 16924, 332507, 6944594, 156127796, 3783620060, 98614428186, 2754950165519, 82200735013648, 2610496155216456, 87952061484214504, 3134296573781405311, 117816169705165137052, 4659486136831587519216, 193431371632142599974128 (list; graph; refs; listen; history; text; internal format)
 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

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Last modified March 23 04:41 EDT 2023. Contains 361434 sequences. (Running on oeis4.)