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A362649
Number of nonisomorphic unordered pairs of involutions on an n-set.
4
1, 1, 3, 4, 10, 13, 27, 36, 69, 92, 162, 217, 365, 487, 782, 1043, 1622, 2154, 3253, 4306, 6355, 8376, 12107, 15893, 22582, 29512, 41285, 53729, 74164, 96100, 131054, 169110, 228165, 293202, 391752, 501406, 664093, 846642, 1112363, 1412768, 1842620
OFFSET
0,3
COMMENTS
Isomorphism is up to permutation of the elements of the n-set.
LINKS
EXAMPLE
The a(4)=10 pairs of involutions are:
{(), ()},
{(), (34)},
{(), (12)(34)},
{(34), (34)},
{(34), (23)},
{(34), (12)},
{(34), (12)(34)},
{(34), (13)(24)},
{(12)(34), (12)(34)},
{(12)(34), (13)(24)}.
CROSSREFS
Column k=2 of A362648.
Sequence in context: A257494 A302347 A092119 * A143372 A035594 A167273
KEYWORD
nonn
AUTHOR
Andrew Howroyd, May 01 2023
STATUS
approved