Magma (algebra)
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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 equipped with a single binary operation . 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 (the English word is a translation of 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 in 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 non necessarily associative binary operation. The word groupoid is used by many universal algebraists, but workers in category theory and related areas object strongly this usage because they use 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 matched with an operation "" that sends any two elements to another element . The symbol "" is a general placeholder for a properly defined operation. To qualify as a magma, the set and operation must satisfy the following requirement (known as the magma axiom):
And in mathematical notation:
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;
 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 nonempty associative quasigroups;
 abelian groups—groups where the operation is commutative.

 Note that each of divisibility and invertibility
 imply the cancellation property.
Morphism of magmas
A morphism of magmas is a function mapping magma to magma , that preserves the binary operation:
where and denote the binary operation on and respectively.
Combinatorics and parentheses
For the general, nonassociative case, the magma operation may be repeatedly iterated. To denote pairings, parentheses are used. The resulting string consists of symbols denoting elements of the magma, and balanced sets of parenthesis. The set of all possible strings of balanced parenthesis 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)).
A shorthand is often used to reduce the number of parentheses. This is accomplished by using juxtaposition in place of the operation. For example, if the magma operation is •, then xy • z abbreviates (x • y) • z. For more complex expressions the use of parentheses is reduced rather than eliminated, as in (a(bc))d = (a • bc)d. A way to avoid completely the use of parentheses is prefix notation.
The number of nonisomorphic magmas having 1, 2, 3, 4, ... elements are 1, 10, 3330, 178981952, ... (sequence A001329 in OEIS). The corresponding numbers of nonisomorphic and nonantiisomorphic magmas are 1, 7, 1734, 89521056, ... (sequence A001424 in OEIS).^{[5]}
Free magma
A free magma on a set is the "most general possible" magma generated by the set (i.e., there are no relations or axioms imposed on the generators; see free object). It can be described as the set of nonassociative words on X with parentheses retained:^{[6]}
It can also be viewed, in terms familiar in computer science, as the magma of binary trees with leaves labelled by elements of . 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 is a function from the set to any magma , then there is a unique extension of to a morphism of magmas
See also: free semigroup, free group, Hall set, Wedderburn–Etherington number
Classification by properties
Template:Grouplike structures A magma (S, •) is called
 medial if it satisfies the identity xy • uz = xu • yz (i.e. (x • y) • (u • z) = (x • u) • (y • z) for all x, y, u, z in S)
 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^{[7]}
 alternative if it satisfies the identities xx • y = x • xy and x • yy = xy • y
 a semigroup if it satisfies the identity x • yz = xy • z (associativity)
 a left unar if it satisfies the identity xy = xz
 a right unar if it satisfies the identity yx = zx
 a semigroup with zero multiplication or a null semigroup if it satisfies the identity xy = uv
 {{safesubst:#invoke:anchormain}}unital if it has an identity element
 powerassociative if the submagma generated by any element is associative,
 leftcancellative if for all x, y, and z, xy = xz implies y = z
 rightcancellative if for all x, y, and z, yx = zx implies y = z
 cancellative if it is both rightcancellative and leftcancellative
 a semigroup with left zeros if it is a semigroup and there are elements x for which the identity x = xy holds
 a semigroup with right zeros if it is a semigroup and there are elements x for which 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.Template:Cn
If • is instead a partial operation, then S is called a partial magma^{[8]} or more often a partial groupoid.^{[8]}^{[9]}
Generalizations
See nary group.
See also
 Magma category
 Auto magma object
 Universal algebra
 Magma computer algebra system, named after the object of this article.
 An example of a commutative nonassociative magma
 Algebraic structures whose axioms are all identities
 Groupoid algebra

References
 ↑ B. A. Hausmann and O ore. Theory of quasigroups", AJM 59(4) 9831004, http://www.jstor.org/stable/2371362
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Further reading
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