A320333 - OEIS

%I #18 Feb 06 2021 15:22:51

%S 1,9,145,4641,231417,16770369,1671395713

%N a(n) is the number of pairings of the first 2*k positive integers into k pairs (xi, yi) such that the k numbers (xi + yi) mod k are all different where k = 2*n + 1.

%C It can be seen as interesting that a(t) = A178185(t+1) for 0 <= t <= 3.

%e a(1) = 9 because {(1,4), (2,5), (3,6)}, {(1,5), (2,6), (3,4)}, {(1,6), (2,4), (3,5)}, {(1,3), (2,6), (4,5)}, {(1,5), (2,3), (4,6)}, {(1,6), (2,3), (4,5)}, {(1,3), (2,4), (5,6)}, {(1,2), (3,5), (4,6)}, {(1,2), (3,4), (5,6)} are corresponding sets.

%Y Cf. A060963, A178185, A320129.

%K nonn,more

%O 0,2

%A _Altug Alkan_, Oct 11 2018