This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A320333 #18 Feb 06 2021 15:22:51
%S A320333 1,9,145,4641,231417,16770369,1671395713
%N A320333 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 A320333 It can be seen as interesting that a(t) = A178185(t+1) for 0 <= t <= 3.
%e A320333 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 A320333 Cf. A060963, A178185, A320129.
%K A320333 nonn,more
%O A320333 0,2
%A A320333 _Altug Alkan_, Oct 11 2018