A362760 Number of nonisomorphic unordered pairs of derangements of an n-set.

1, 0, 1, 2, 7, 16, 84, 403, 3028, 25431, 250377, 2726361, 32622807, 423310642, 5921052187, 88759485250, 1419511438134, 24123164524402, 434094104795638, 8245872981392311, 164885609163058430, 3462034812141768953, 76154237902292661820, 1751339843001023621169

Offset

0,4

Comments

Isomorphism is up to permutation of the elements of the n-set.

Links

Andrew Howroyd, Table of n, a(n) for n = 0..50

Example

The a(3)=2 nonisomorphic pairs of derangements with permutations shown in cycle notation are:
  {(123), (123)},
  {(123), (132)}.
The a(4)=7 nonisomorphic pairs of derangements are:
  {(12)(34), (12)(34)},
  {(12)(34), (13)(24)},
  {(12)(34), (1234)},
  {(12)(34), (1324)},
  {(1234), (1234)},
  {(1234), (1243)},
  {(1234), (1432)}.

Crossrefs

Column k=2 of A362759.

Cf. A000166 (derangements), A362645.

Sequence in context: A073998 A129444 A079815 * A325510 A347601 A006883

Adjacent sequences: A362757 A362758 A362759 * A362761 A362762 A362763

Keyword

nonn

Author

Andrew Howroyd, May 02 2023

Status

approved