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 A362649 #8 May 03 2023 10:51:56
%S A362649 1,1,3,4,10,13,27,36,69,92,162,217,365,487,782,1043,1622,2154,3253,
%T A362649 4306,6355,8376,12107,15893,22582,29512,41285,53729,74164,96100,
%U A362649 131054,169110,228165,293202,391752,501406,664093,846642,1112363,1412768,1842620
%N A362649 Number of nonisomorphic unordered pairs of involutions on an n-set.
%C A362649 Isomorphism is up to permutation of the elements of the n-set.
%H A362649 Andrew Howroyd, <a href="/A362649/b362649.txt">Table of n, a(n) for n = 0..50</a>
%e A362649 The a(4)=10 pairs of involutions are:
%e A362649 {(), ()},
%e A362649 {(), (34)},
%e A362649 {(), (12)(34)},
%e A362649 {(34), (34)},
%e A362649 {(34), (23)},
%e A362649 {(34), (12)},
%e A362649 {(34), (12)(34)},
%e A362649 {(34), (13)(24)},
%e A362649 {(12)(34), (12)(34)},
%e A362649 {(12)(34), (13)(24)}.
%Y A362649 Column k=2 of A362648.
%Y A362649 Cf. A000085, A362645.
%K A362649 nonn
%O A362649 0,3
%A A362649 _Andrew Howroyd_, May 01 2023