A390163 Number of nonisomorphic semigroups with n elements satisfying the equation xyz = xy.

1, 1, 2, 3, 6, 14, 47, 194, 1153, 10525, 164430, 4731263

Offset

0,3

Comments

A semigroup is an associative magma.

Links

Table of n, a(n) for n=0..11.

Index entries for sequences related to semigroups.

Example

The a(4) = 6 semigroups given by their Cayley tables are:
  1 1 1 1   1 1 1 1   1 1 1 1   1 1 1 1   1 1 1 1   1 1 1 1
  1 1 1 1   1 1 1 1   1 1 1 1   1 1 1 1   1 1 1 3   2 2 2 2
  1 1 1 1   1 1 1 1   3 3 3 3   3 3 3 3   3 3 3 3   3 3 3 3
  1 1 1 1   4 4 4 4   3 3 3 3   4 4 4 4   4 4 4 4   4 4 4 4

Crossrefs

Cf. A027851, A362385, A390162, A390165 (labeled case).

Sequence in context: A091285 A109459 A118986 * A091138 A335260 A090983

Adjacent sequences: A390160 A390161 A390162 * A390164 A390165 A390166

Keyword

nonn,more

Author

Andrew Howroyd, Nov 18 2025

Status

approved