IIn abstract algebra, a magma (or groupoid; not to be confused with groupoids in category theory) is a basic kind of algebraic structure. Specifically, a magma consists of a set equipped with a single binary operation that must be closed by definition. No other properties are imposed.

History and terminology

The term groupoid was introduced in 1927 by Heinrich Brandt describing his Brandt groupoid (translated from the German Gruppoid). The term was then appropriated by B. A. Hausmann and Øystein Ore (1937)[1] in the sense (of a set with a binary operation) used in this article. In a couple of reviews of subsequent papers in Zentralblatt, Brandt strongly disagreed with this overloading of terminology. The Brandt groupoid is a groupoid in the sense used in category theory, but not in the sense used by Hausmann and Ore. Nevertheless, influential books in semigroup theory, including Clifford and Preston (1961) and Howie (1995) use groupoid in the sense of Hausmann and Ore. Hollings (2014) writes that the term groupoid is "perhaps most often used in modern mathematics" in the sense given to it in category theory.[2]

According to Bergman and Hausknecht (1996): "There is no generally accepted word for a set with a not necessarily associative binary operation. The word groupoid is used by many universal algebraists, but workers in category theory and related areas object strongly to this usage because they use the same word to mean 'category in which all morphisms are invertible'. The term magma was used by Serre [Lie Algebras and Lie Groups, 1965]."[3] It also appears in Bourbaki's Éléments de mathématique, Algèbre, chapitres 1 à 3, 1970.[4]

Definition

A magma is a set M matched with an operation, •, that sends any two elements a, b ∈ M to another element, a • b. The symbol, •, is a general placeholder for a properly defined operation. To qualify as a magma, the set and operation (M, •) must satisfy the following requirement (known as the magma or closure axiom):

For all a, b in M, the result of the operation a • b is also in M.

And in mathematical notation:

\( {\displaystyle a,b\in M\implies a\cdot b\in M}. \)

If • is instead a partial operation, then S is called a partial magma[5] or more often a partial groupoid.[5][6]

Morphism of magmas

A morphism of magmas is a function, f : M → N, mapping magma M to magma N, that preserves the binary operation:

f (x •M y) = f(x) •N f(y)

where •M and •N denote the binary operation on M and N respectively.

Notation and combinatorics

The magma operation may be applied repeatedly, and in the general, non-associative case, the order matters, which is notated with parentheses. Also, the operation, •, is often omitted and notated by juxtaposition:

(a • (b • c)) • d = (a(bc))d

A shorthand is often used to reduce the number of parentheses, in which the innermost operations and pairs of parentheses are omitted, being replaced just with juxtaposition, xy • z = (x • y) • z. For example, the above is abbreviated to the following expression, still containing parentheses:

(a • bc)d.

A way to avoid completely the use of parentheses is prefix notation, in which the same expression would be written ••a•bcd. Another way, familiar to programmers, is postfix notation (Reverse Polish notation), in which the same expression would be written abc••d•, in which the order of execution is simply left-to-right (no Currying).

The set of all possible strings consisting of symbols denoting elements of the magma, and sets of balanced parentheses is called the Dyck language. The total number of different ways of writing n applications of the magma operator is given by the Catalan number, Cn. Thus, for example, C2 = 2, which is just the statement that (ab)c and a(bc) are the only two ways of pairing three elements of a magma with two operations. Less trivially, C3 = 5: ((ab)c)d, (a(bc))d, (ab)(cd), a((bc)d), and a(b(cd)).

There are \( {\displaystyle n^{n^{2}}} \) magmas with n elements so there are 1, 1, 16, 19683, 4294967296, ... (sequence A002489 in the OEIS) magmas with 0, 1, 2, 3, 4, ... elements. The corresponding numbers of non-isomorphic magmas are 1, 1, 10, 3330, 178981952, ... (sequence A001329 in the OEIS) and the numbers of simultaneously non-isomorphic and non-antiisomorphic magmas are 1, 1, 7, 1734, 89521056, ... (sequence A001424 in the OEIS).[7]

Free magma

A free magma, MX, on a set, X, is the "most general possible" magma generated by X (i.e., there are no relations or axioms imposed on the generators; see free object). It can be described as the set of non-associative words on X with parentheses retained.[8]

It can also be viewed, in terms familiar in computer science, as the magma of binary trees with leaves labelled by elements of X. The operation is that of joining trees at the root. It therefore has a foundational role in syntax.

A free magma has the universal property such that, if f : X → N is a function from X to any magma, N, then there is a unique extension of f to a morphism of magmas, f ′

f ′ : MX → N.

See also: Free semigroup, Free group, Hall set, and Wedderburn–Etherington number

Types of magma

Magmas are not often studied as such; instead there are several different kinds of magma, depending on what axioms the operation is required to satisfy. Commonly studied types of magma include:

Quasigroup

A magma where division is always possible

Loop

A quasigroup with an identity element

Semigroup

A magma where the operation is associative

Inverse semigroup

A semigroup with inverse.

Semilattice

A semigroup where the operation is commutative and idempotent

Monoid

A semigroup with an identity element

Group

A monoid with inverse elements, or equivalently, an associative loop, or a non-empty associative quasigroup

Abelian group

A group where the operation is commutative

Note that each of divisibility and invertibility imply the cancellation property.

Classification by properties

Group-like structures | |||||
---|---|---|---|---|---|

Totality^{α} |
Associativity | Identity | Invertibility | Commutativity | |

Semigroupoid | Unneeded | Required | Unneeded | Unneeded | Unneeded |

Small Category | Unneeded | Required | Required | Unneeded | Unneeded |

Groupoid | Unneeded | Required | Required | Required | Unneeded |

Magma | Required | Unneeded | Unneeded | Unneeded | Unneeded |

Quasigroup | Required | Unneeded | Unneeded | Required | Unneeded |

Unital Magma | Required | Unneeded | Required | Unneeded | Unneeded |

Loop | Required | Unneeded | Required | Required | Unneeded |

Semigroup | Required | Required | Unneeded | Unneeded | Unneeded |

Inverse Semigroup | Required | Required | Unneeded | Required | Unneeded |

Monoid | Required | Required | Required | Unneeded | Unneeded |

Commutative monoid | Required | Required | Required | Unneeded | Required |

Group | Required | Required | Required | Required | Unneeded |

Abelian group | Required | Required | Required | Required | Required |

^α Closure, which is used in many sources, is an equivalent axiom to totality, though defined differently. |

A magma (S, •), with x, y, u, z ∈ S, is called

Medial

If it satisfies the identity, xy • uz ≡ xu • yz

Left semimedial

If it satisfies the identity, xx • yz ≡ xy • xz

Right semimedial

If it satisfies the identity, yz • xx ≡ yx • zx

Semimedial

If it is both left and right semimedial

Left distributive

If it satisfies the identity, x • yz ≡ xy • xz

Right distributive

If it satisfies the identity, yz • x ≡ yx • zx

Autodistributive

If it is both left and right distributive

Commutative

If it satisfies the identity, xy ≡ yx

Idempotent

If it satisfies the identity, xx ≡ x

Unipotent

If it satisfies the identity, xx ≡ yy

Zeropotent

If it satisfies the identities, xx • y ≡ xx ≡ y • xx[9]

Alternative

If it satisfies the identities xx • y ≡ x • xy and x • yy ≡ xy • y

Power-associative

If the submagma generated by any element is associative

Flexible

if xy • x ≡ x • yx

A semigroup, or associative

If it satisfies the identity, x • yz ≡ xy • z

A left unar

If it satisfies the identity, xy ≡ xz

A right unar

If it satisfies the identity, yx ≡ zx

Semigroup with zero multiplication, or null semigroup

If it satisfies the identity, xy ≡ uv

Unital

If it has an identity element

Left-cancellative

If, for all x, y, and, z, xy = xz implies y = z

Right-cancellative

If, for all x, y, and, z, yx = zx implies y = z

Cancellative

If it is both right-cancellative and left-cancellative

A semigroup with left zeros

If it is a semigroup and, for all x, the identity, x ≡ xy, holds

A semigroup with right zeros

If it is a semigroup and, for all x, the identity, x ≡ yx, holds

Trimedial

If any triple of (not necessarily distinct) elements generates a medial submagma

Entropic

If it is a homomorphic image of a medial cancellation magma.[10]

Generalizations

See n-ary group.

See also

Magma category

Auto magma object

Universal algebra

Magma computer algebra system, named after the object of this article.

Commutative non-associative magmas

Algebraic structures whose axioms are all identities

Groupoid algebra

Hall set

References

Hausmann, B. A.; Ore, Øystein (October 1937), "Theory of quasi-groups", American Journal of Mathematics, 59 (4): 983–1004, doi:10.2307/2371362, JSTOR 2371362

Hollings, Christopher (2014), Mathematics across the Iron Curtain: A History of the Algebraic Theory of Semigroups, American Mathematical Society, pp. 142–3, ISBN 978-1-4704-1493-1

Bergman, George M.; Hausknecht, Adam O. (1996), Cogroups and Co-rings in Categories of Associative Rings, American Mathematical Society, p. 61, ISBN 978-0-8218-0495-7

Bourbaki, N. (1998) [1970], "Algebraic Structures: §1.1 Laws of Composition: Definition 1", Algebra I: Chapters 1–3, Springer, p. 1, ISBN 978-3-540-64243-5

Müller-Hoissen, Folkert; Pallo, Jean Marcel; Stasheff, Jim, eds. (2012), Associahedra, Tamari Lattices and Related Structures: Tamari Memorial Festschrift, Springer, p. 11, ISBN 978-3-0348-0405-9

Evseev, A. E. (1988), "A survey of partial groupoids", in Silver, Ben (ed.), Nineteen Papers on Algebraic Semigroups, American Mathematical Society, ISBN 0-8218-3115-1

Weisstein, Eric W. "Groupoid". MathWorld.

Rowen, Louis Halle (2008), "Definition 21B.1.", Graduate Algebra: Noncommutative View, Graduate Studies in Mathematics, American Mathematical Society, p. 321, ISBN 0-8218-8408-5

Kepka, T.; Němec, P. (1996), "Simple balanced groupoids" (PDF), Acta Universitatis Palackianae Olomucensis. Facultas Rerum Naturalium. Mathematica, 35 (1): 53–60

Ježek, Jaroslav; Kepka, Tomáš (1981), "Free entropic groupoids" (PDF), Commentationes Mathematicae Universitatis Carolinae, 22 (2): 223–233, MR 0620359.

M. Hazewinkel (2001) [1994], "Magma", Encyclopedia of Mathematics, EMS Press

M. Hazewinkel (2001) [1994], "Groupoid", Encyclopedia of Mathematics, EMS Press

M. Hazewinkel (2001) [1994], "Free magma", Encyclopedia of Mathematics, EMS Press

Weisstein, Eric W. "Groupoid". MathWorld.

Further reading

Bruck, Richard Hubert (1971), A survey of binary systems (3rd ed.), Springer, ISBN 978-0-387-03497-3

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