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%I #6 May 03 2023 21:36:58
%S 1,0,1,2,7,16,84,403,3028,25431,250377,2726361,32622807,423310642,
%T 5921052187,88759485250,1419511438134,24123164524402,434094104795638,
%U 8245872981392311,164885609163058430,3462034812141768953,76154237902292661820,1751339843001023621169
%N Number of nonisomorphic unordered pairs of derangements of an n-set.
%C Isomorphism is up to permutation of the elements of the n-set.
%H Andrew Howroyd, <a href="/A362760/b362760.txt">Table of n, a(n) for n = 0..50</a>
%e The a(3)=2 nonisomorphic pairs of derangements with permutations shown in cycle notation are:
%e {(123), (123)},
%e {(123), (132)}.
%e The a(4)=7 nonisomorphic pairs of derangements are:
%e {(12)(34), (12)(34)},
%e {(12)(34), (13)(24)},
%e {(12)(34), (1234)},
%e {(12)(34), (1324)},
%e {(1234), (1234)},
%e {(1234), (1243)},
%e {(1234), (1432)}.
%Y Column k=2 of A362759.
%Y Cf. A000166 (derangements), A362645.
%K nonn
%O 0,4
%A _Andrew Howroyd_, May 02 2023
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