A320168 Number of ways to group the first 2*n positive integers into n pairs (xi, yi) with xi < yi, and such that the n numbers (yi mod xi) are all different.

1, 1, 2, 2, 7, 12, 22, 26, 85, 226, 717, 1695, 5071, 14275, 47405, 176747, 638329, 2166516

Offset

0,3

Comments

How does a(n+1)/a(n) behave as n increases?

Links

Table of n, a(n) for n=0..17.

David A. Corneth, Illustration for a(4)

Example

a(3) = 2 because {(1,3), (2,5), (4,6)} and {(1,5), (2,3), (4,6)} are corresponding sets.
a(4) = 7 because {(1,6), (2,5), (3,8), (4,7)}, {(1,3), (2,7), (4,6), (5,8)}, {(1,7), (2,3), (4,6), (5,8)}, {(1,3), (2,5), (4,7), (6,8)}, {(1,5), (2,3), (4,7), (6,8)}, {(1,2), (3,7), (4,6), (5,8)}, {(1,2), (3,8), (4,7), (5,6)} are corresponding sets.

Crossrefs

Cf. A002968, A060963, A272363, A320129.

Sequence in context: A005298 A063692 A292615 * A123209 A123604 A321306

Adjacent sequences: A320165 A320166 A320167 * A320169 A320170 A320171

Keyword

nonn,more

Author

Altug Alkan, Oct 07 2018

Extensions

a(13)-a(17) from Rémy Sigrist, Oct 07 2018

Status

approved