cp's OEIS Frontend

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.

%I A190825 #15 Oct 26 2017 03:21:54
%S A190825 1,0,0,0,0,1,40,876,16924,332507,6944594,156127796,3783620060,
%T A190825 98614428186,2754950165519,82200735013648,2610496155216456,
%U A190825 87952061484214504,3134296573781405311,117816169705165137052,4659486136831587519216,193431371632142599974128
%N 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.
%H A190825 Alois P. Heinz, <a href="/A190825/b190825.txt">Table of n, a(n) for n = 0..404</a>
%H A190825 Everett Sullivan, <a href="https://arxiv.org/abs/1611.02771">Linear chord diagrams with long chords</a>, arXiv preprint arXiv:1611.02771 [math.CO], 2016.
%F A190825 a(n) ~ 2^(n + 1/2) * n^n / exp(n+4). - _Vaclav Kotesovec_, Oct 26 2017
%e A190825 Some solutions for n=6
%e A190825 ..1....1....1....1....1....1....1....1....1....1....1....1....1....1....1....1
%e A190825 ..2....2....2....2....2....2....2....2....2....2....2....2....2....2....2....2
%e A190825 ..3....3....3....3....3....3....3....3....3....3....3....3....3....3....3....3
%e A190825 ..4....4....4....4....4....4....4....4....4....4....4....4....4....4....4....4
%e A190825 ..5....5....5....5....5....5....5....5....5....5....5....5....5....5....5....5
%e A190825 ..6....6....6....6....6....6....6....6....6....6....1....1....1....6....6....1
%e A190825 ..1....2....1....2....1....1....2....2....1....2....6....6....6....1....2....6
%e A190825 ..2....3....3....3....3....2....3....3....3....3....3....2....2....3....1....2
%e A190825 ..3....1....2....1....4....3....1....4....2....4....4....4....3....4....4....4
%e A190825 ..5....4....5....5....5....4....5....5....4....1....2....3....5....5....5....5
%e A190825 ..4....6....6....4....2....6....6....1....6....5....5....5....4....6....3....3
%e A190825 ..6....5....4....6....6....5....4....6....5....6....6....6....6....2....6....6
%Y A190825 Column k=5 of A293157.
%K A190825 nonn
%O A190825 0,7
%A A190825 _R. H. Hardin_, May 21 2011
%E A190825 a(0), a(18)-a(21) from _Alois P. Heinz_, Oct 17 2017