In mathematics, a semigroup with two elements is a semigroup for which the cardinality of the underlying set is two. There are exactly five nonisomorphic semigroups having two elements:

• O₂, the null semigroup of order two,
• LO₂, the left zero semigroup of order two,
• RO₂, the right zero semigroup of order two,
• ({0,1}, ∧) (where "∧" is the logical connective "and"), or equivalently the set {0,1} under multiplication: the only semilattice with two elements and the only non-null semigroup with zero of order two, also a monoid, and ultimately the two-element Boolean algebra,
• (Z₂, +₂) (where Z₂ = {0,1} and "+₂" is "addition modulo 2"), or equivalently ({0,1}, ⊕) (where "⊕" is the logical connective "xor"), or equivalently the set {−1,1} under multiplication: the only group of order two. This is also isomorphic to (Z₂, ·₂), the multiplicative group of {0,1} modulo 2.

The semigroups LO₂ and RO₂ are antiisomorphic. O₂, ({0,1}, ∧) and (Z₂, +₂) are commutative, and LO₂ and RO₂ are noncommutative. LO₂, RO₂ and ({0,1}, ∧) are bands.

Determination of semigroups with two elements

Choosing the set A = { 1, 2 } as the underlying set having two elements, sixteen binary operations can be defined in A. These operations are shown in the table below. In the table, a matrix of the form

indicates a binary operation on A having the following Cayley table.

List of binary operations in { 1, 2 }

1    1    1    1	1    1    1    2	1    1    2    1	1    1    2    2
Null semigroup O2	≡ Semigroup ({0,1}, ${\displaystyle \wedge }$)	2·(1·2) = 2, (2·1)·2 = 1	Left zero semigroup LO2
1    2    1    1	1    2    1    2	1    2    2    1	1    2    2    2
2·(1·2) = 1, (2·1)·2 = 2	Right zero semigroup RO2	≡ Group (Z2, ·2)	≡ Semigroup ({0,1}, ${\displaystyle \wedge }$)
2    1    1    1	2    1    1    2	2    1    2    1	2    1    2    2
1·(1·2) = 2, (1·1)·2 = 1	≡ Group (Z2, +2)	1·(1·1) = 1, (1·1)·1 = 2	1·(2·1) = 1, (1·2)·1 = 2
2    2    1    1	2    2    1    2	2    2    2    1	2    2    2    2
1·(1·1) = 2, (1·1)·1 = 1	1·(2·1) = 2, (1·2)·1 = 1	1·(1·2) = 2, (1·1)·2 = 1	Null semigroup O2

In this table:

• The semigroup ({0,1}, ${\displaystyle \wedge }$) denotes the two-element semigroup containing the zero element 0 and the unit element 1. The two binary operations defined by matrices in a green background are associative and pairing either with A creates a semigroup isomorphic to the semigroup ({0,1}, ${\displaystyle \wedge }$). Every element is idempotent in this semigroup, so it is a band. Furthermore, it is commutative (abelian) and thus a semilattice. The order induced is a linear order, and so it is in fact a lattice and it is also a distributive and complemented lattice, i.e. it is actually the two-element Boolean algebra.
• The two binary operations defined by matrices in a blue background are associative and pairing either with A creates a semigroup isomorphic to the null semigroup O₂ with two elements.
• The binary operation defined by the matrix in an orange background is associative and pairing it with A creates a semigroup. This is the left zero semigroup LO₂. It is not commutative.
• The binary operation defined by the matrix in a purple background is associative and pairing it with A creates a semigroup. This is the right zero semigroup RO₂. It is also not commutative.
• The two binary operations defined by matrices in a red background are associative and pairing either with A creates a semigroup isomorphic to the group (Z₂, +₂).
• The remaining eight binary operations defined by matrices in a white background are not associative and hence none of them create a semigroup when paired with A.

The two-element semigroup ({0,1}, ∧)

The Cayley table for the semigroup ({0,1}, ${\displaystyle \wedge }$) is given below:

${\displaystyle \wedge }$	0	1
0	0	0
1	0	1

This is the simplest non-trivial example of a semigroup that is not a group. This semigroup has an identity element, 1, making it a monoid. It is also commutative. It is not a group because the element 0 does not have an inverse, and is not even a cancellative semigroup because we cannot cancel the 0 in the equation 1·0 = 0·0.

This semigroup arises in various contexts. For instance, if we choose 1 to be the truth value "true" and 0 to be the truth value "false" and the operation to be the logical connective "and", we obtain this semigroup in logic. It is isomorphic to the monoid {0,1} under multiplication. It is also isomorphic to the semigroup

${\displaystyle S=\left\{{\begin{pmatrix}1&0\\0&1\end{pmatrix}},{\begin{pmatrix}1&0\\0&0\end{pmatrix}}\right\}}$

under matrix multiplication.

The two-element semigroup (Z₂, +₂)

The Cayley table for the semigroup (Z₂, +₂) is given below:

This group is isomorphic to the cyclic group Z₂ and the symmetric group S₂.

Semigroups of order 3

Let A be the three-element set {1, 2, 3}. Altogether, a total of 3⁹ = 19683 different binary operations can be defined on A. 113 of the 19683 binary operations determine 24 nonisomorphic semigroups, or 18 non-equivalent semigroups (with equivalence being isomorphism or anti-isomorphism). ^[1] With the exception of the group with three elements, each of these has one (or more) of the above two-element semigroups as subsemigroups.^[2] For example, the set {−1, 0, 1} under multiplication is a semigroup of order 3, and contains both {0, 1} and {−1, 1} as subsemigroups.

Finite semigroups of higher orders

Algorithms and computer programs have been developed for determining nonisomorphic finite semigroups of a given order. These have been applied to determine the nonisomorphic semigroups of small order.^[2][3][4] The number of nonisomorphic semigroups with n elements, for n a nonnegative integer, is listed under OEIS: A027851 in the On-Line Encyclopedia of Integer Sequences. OEIS: A001423 lists the number of non-equivalent semigroups, and OEIS: A023814 the number of associative binary operations, out of a total of nⁿ², determining a semigroup.

See also

• Empty semigroup
• Trivial semigroup (semigroup with one element)
• Semigroup with three elements
• Special classes of semigroups

References