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 A006200 M4263 #21 Sep 25 2018 21:12:01
%S A006200 1,6,55,610,7980,120274,2052309,39110490,823324755,18974858540,
%T A006200 475182478056,12848667150956,373081590628565,11578264139795430,
%U A006200 382452947343624515,13397354334102974934,496082324933446766724,19360538560004548357830,794275868644522931369185
%N A006200 Number of partitions into pairs.
%D A006200 G. Kreweras and Y. Poupard, Sur les partitions en paires d'un ensemble fini totalement ordonne, Publications de l'Institut de Statistique de l'Université de Paris, 23 (1978), 57-74.
%D A006200 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%F A006200 a(n) = A079267(n + 2, 3). - _Sean A. Irvine_, Jan 24 2017
%F A006200 a(n) ~ 2^(n + 3/2) * n^(n + 2) / (3 * exp(n + 1)). - _Vaclav Kotesovec_, May 20 2018
%p A006200 a:= proc(n) option remember; `if`(n<2, n,
%p A006200 (n*(4*n^2-7)*a(n-1)+(n+1)*(2*n+1)*a(n-2))/((2*n-1)*(n-1)))
%p A006200 end:
%p A006200 seq(a(n), n=1..20); # _Alois P. Heinz_, Jan 24 2017
%t A006200 Table[(2*n+1)! * Hypergeometric1F1[1-n, -1-2*n, -2] / (3*2^n*(n-1)!), {n, 1, 20}] (* _Vaclav Kotesovec_, Jan 24 2017 *)
%Y A006200 Cf. A079267.
%K A006200 nonn
%O A006200 1,2
%A A006200 _N. J. A. Sloane_
%E A006200 More terms from _Sean A. Irvine_, Jan 24 2017