Magma (algebra)

In 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, M, equipped with a single binary operation, M × M → M. The binary operation must be closed by definition but no other properties are imposed.

History and terminology

The term groupoid was introduced in 1926 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:

∀ a, b ∈ M: a • b ∈ 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 $•• a • bcd$ .

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, $C n$ . Thus, for example, $C 2 = 2$ , 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, $C 3 = 5$ : $((ab) c) d$ , $(a (bc)) d$ , $(ab)(cd)$ , $a ((bc) d)$ , and $a (b (cd))$ .

The number of non-isomorphic magmas having 0, 1, 2, 3, 4, ... elements are 1, 1, 10, 3330, 178981952, ... (sequence A001329 in the OEIS). The corresponding numbers of non-isomorphic and non-antiisomorphic magmas are 1, 1, 7, 1734, 89521056, ... (sequence A001424 in the OEIS).^[7]

Free magma

A free magma, M_X, 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 ′ : M_X → N.

Types of magmas

Magmas are not often studied as such; instead there are several different kinds of magmas, depending on what axioms one might require of the operation. Commonly studied types of magmas include:

Quasigroups: Magmas where division is always possible
Loops: Quasigroups with identity elements

AG-groupoids: Magmas satisfy left invertive law

AG-monoids: AG-groupoid with left identity
AG-groups: AG-monoid with inverse elements
Semigroups: Magmas where the operation is associative
Semilattices: Semigroups where the operation is commutative and idempotent
Monoids: Semigroups with identity elements
Groups: Monoids with inverse elements, or equivalently, associative loops or non-empty associative quasigroups
Abelian groups: Groups where the operation is commutative

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

Classification by properties

Group-like structures
	Total	Associative	Identity	Cancellation	Commutative
Partial magma	Unneeded	Unneeded	Unneeded	Unneeded	Unneeded
Semigroupoid	Unneeded	Required	Unneeded	Unneeded	Unneeded
Small category	Unneeded	Required	Required	Unneeded	Unneeded
Groupoid	Unneeded	Required	Required	Required	Unneeded
Commutative Groupoid	Unneeded	Required	Required	Required	Required
Magma	Required	Unneeded	Unneeded	Unneeded	Unneeded
Commutative magma	Required	Unneeded	Unneeded	Unneeded	Required
Quasigroup	Required	Unneeded	Unneeded	Required	Unneeded
Commutative quasigroup	Required	Unneeded	Unneeded	Required	Required
Unital magma	Required	Unneeded	Required	Unneeded	Unneeded
Commutative unital magma	Required	Unneeded	Required	Unneeded	Required
Loop	Required	Unneeded	Required	Required	Unneeded
Commutative loop	Required	Unneeded	Required	Required	Required
Semigroup	Required	Required	Unneeded	Unneeded	Unneeded
Commutative semigroup	Required	Required	Unneeded	Unneeded	Required
Associative quasigroup	Required	Required	Unneeded	Required	Unneeded
Commutative-and-associative quasigroup	Required	Required	Unneeded	Required	Required
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

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

Medial: If it satisfies the identity, $xy • uz \equiv xu • yz$
Left semimedial: If it satisfies the identity, $xx • yz \equiv xy • xz$
Right semimedial: If it satisfies the identity, $yz • xx \equiv yx • zx$
Semimedial: If it is both left and right semimedial
Left distributive: If it satisfies the identity, $x • yz \equiv xy • xz$
Right distributive: If it satisfies the identity, $yz • x \equiv yx • zx$
Autodistributive: If it is both left and right distributive
Commutative: If it satisfies the identity, $xy \equiv yx$
Idempotent: If it satisfies the identity, $xx \equiv x$
Unipotent: If it satisfies the identity, $xx \equiv yy$
Zeropotent: If it satisfies the identities, $xx • y \equiv xx \equiv y • xx$ ^[9]
Alternative: If it satisfies the identities $xx • y \equiv x • xy$ and $x • yy \equiv xy • y$
Power-associative: If the submagma generated by any element is associative
A semigroup, or associative: If it satisfies the identity, $x • yz \equiv xy • z$
A left unar: If it satisfies the identity, $xy \equiv xz$
A right unar: If it satisfies the identity, $yx \equiv zx$
Semigroup with zero multiplication, or null semigroup: If it satisfies the identity, $xy \equiv uv$
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, $x \equiv xy$ , holds
A semigroup with right zeros: If it is a semigroup and, for all $x$ , the identity, $x \equiv yx$ , 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.

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{{citation}}: CS1 maint: year (link)
^ 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 {{citation}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)
^ ^a ^b 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 {{citation}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)
^ 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.