In abstract algebra, a branch of mathematics, a monoid is a set equipped with an associative binary operation and an identity element. Monoids are semigroups with identity. Such algebraic structures occur in several branches of mathematics. For example, the functions from a set into itself form a monoid with respect to function composition. More generally, in category theory, the morphisms of an object (category theory), object to itself form a monoid, and, conversely, a monoid may be viewed as a category with a single object. In computer science and computer programming, the set of string (computer science), strings built from a given set of Character (computing), characters is a free monoid. Transition monoids and syntactic monoids are used in describing finite-state machines. Trace monoids and history monoids provide a foundation for process calculi and concurrent computing. In theoretical computer science, the study of monoids is fundamental for automata theory (Krohn–Rhodes theory), and formal language theory (star height problem). See semigroup for the history of the subject, and some other general properties of monoids.


A set (mathematics), set ''S'' equipped with a binary operation , which we will denote •, is a monoid if it satisfies the following two axioms: ; Associativity: For all ''a'', ''b'' and ''c'' in ''S'', the equation holds. ; Identity element: There exists an element ''e'' in ''S'' such that for every element ''a'' in ''S'', the equations and hold. In other words, a monoid is a semigroup with an identity element. It can also be thought of as a magma (algebra), magma with associativity and identity. The identity element of a monoid is unique. For this reason the identity is regarded as a Constant (mathematics), constant, i. e. 0-ary (or nullary) operation. The monoid therefore is characterized by specification of the Tuple, triple (''S'', • , ''e''). Depending on the context, the symbol for the binary operation may be omitted, so that the operation is denoted by juxtaposition; for example, the monoid axioms may be written and . This notation does not imply that it is numbers being multiplied. A monoid in which each element has an inverse element, inverse is a group (mathematics), group.

Monoid structures


A submonoid of a monoid is a subset ''N'' of ''M'' that is closed under the monoid operation and contains the identity element ''e'' of ''M''. Symbolically, ''N'' is a submonoid of ''M'' if , whenever , and . In this case, ''N'' is a monoid under the binary operation inherited from ''M''. On the other hand, if ''N'' is subset of a monoid that is closure (mathematics), closed under the monoid operation, and is a monoid for this inherited operation, then ''N'' is not always a submonoid, since the identity elements may differ. For example, the singleton set is closed under multiplication, and is not a submonoid of the (multiplicative) monoid of the nonnegative integers.


A subset ''S'' of ''M'' is said to ''generate'' ''M'' if the smallest submonoid of ''M'' containing ''S'' is ''M''. If there is a finite set that generates ''M'', then ''M'' is said to be a finitely generated monoid.

Commutative monoid

A monoid whose operation is commutative is called a commutative monoid (or, less commonly, an abelian monoid). Commutative monoids are often written additively. Any commutative monoid is endowed with its ''algebraic'' preordering , defined by if there exists ''z'' such that . An ''order-unit'' of a commutative monoid ''M'' is an element ''u'' of ''M'' such that for any element ''x'' of ''M'', there exists ''v'' in the set generated by ''u'' such that . This is often used in case ''M'' is the Ordered group, positive cone of a Partially ordered set, partially ordered abelian group ''G'', in which case we say that ''u'' is an order-unit of ''G''.

Partially commutative monoid

A monoid for which the operation is commutative for some, but not all elements is a trace monoid; trace monoids commonly occur in the theory of concurrent computation.


* Out of the 16 possible truth table#Truth table for all binary logical operators, binary Boolean operators, each of the four that has a two-sided identity is also commutative and associative and thus makes the set a commutative monoid. Under the standard definitions, Logical conjunction, AND and Logical biconditional, XNOR have the identity True while Exclusive disjunction, XOR and Logical disjunction, OR have the identity False. The monoids from AND and OR are also idempotent while those from XOR and XNOR are not. * The set of natural numbers \N = \ is a commutative monoid under addition (identity element 0 (number), 0) or multiplication (identity element 1 (number), 1). A submonoid of under addition is called a numerical monoid. * The set of positive integers \N \setminus \ is a commutative monoid under multiplication (identity element 1). * Given a set , the set of subsets of is a commutative monoid under intersection (identity element is itself). * Given a set , the set of subsets of is a commutative monoid under union (identity element is the empty set). * Generalizing the previous example, every bounded semilattice is an idempotent commutative monoid. ** In particular, any bounded lattice (order), lattice can be endowed with both a Join and meet, meet- and a Join and meet, join- monoid structure. The identity elements are the lattice's top and its bottom, respectively. Being lattices, Heyting algebras and Boolean algebra (structure), Boolean algebras are endowed with these monoid structures. * Every singleton set closed under a binary operation • forms the trivial (one-element) monoid, which is also the trivial group. * Every group (mathematics), group is a monoid and every abelian group a commutative monoid. * Any semigroup may be turned into a monoid simply by adjoining an element not in and defining for all . This conversion of any semigroup to the monoid is done by the free functor between the category of semigroups and the category of monoids. ** Thus, an idempotent monoid (sometimes known as ''find-first'') may be formed by adjoining an identity element to the left zero semigroup over a set . The opposite monoid (sometimes called ''find-last'') is formed from the right zero semigroup over . *** Adjoin an identity to the left-zero semigroup with two elements . Then the resulting idempotent monoid models the lexicographical order of a sequence given the orders of its elements, with ''e'' representing equality. * The underlying set of any ring (algebra), ring, with addition or multiplication as the operation. (By definition, a ring has a multiplicative identity 1.) ** The integers, rational numbers, real numbers or complex numbers, with addition or multiplication as operation. ** The set of all by matrix (mathematics), matrices over a given ring, with matrix addition or matrix multiplication as the operation. * The set of all finite string (computer science), strings over some fixed alphabet forms a monoid with string concatenation as the operation. The empty string serves as the identity element. This monoid is denoted and is called the ''free monoid'' over . It is not commutative. * Given any monoid , the ''opposite monoid'' has the same carrier set and identity element as , and its operation is defined by . Any commutative monoid is the opposite monoid of itself. * Given two sets and endowed with monoid structure (or, in general, any finite number of monoids, , their cartesian product is also a monoid (respectively, ). The associative operation and the identity element are defined pairwise. * Fix a monoid . The set of all functions from a given set to is also a monoid. The identity element is a constant function mapping any value to the identity of ; the associative operation is defined pointwise. * Fix a monoid with the operation and identity element , and consider its power set consisting of all subsets of . A binary operation for such subsets can be defined by . This turns into a monoid with identity element . In the same way the power set of a group is a monoid under the product of group subsets. * Let be a set. The set of all functions forms a monoid under function composition. The identity is just the identity function. It is also called the ''full transformation monoid'' of . If is finite with elements, the monoid of functions on is finite with elements. * Generalizing the previous example, let be a category (mathematics), category and an object of . The set of all endomorphisms of , denoted , forms a monoid under composition of morphisms. For more on the relationship between category theory and monoids see below. * The set of homeomorphism Class (set theory), classes of compact surfaces with the connected sum. Its unit element is the class of the ordinary 2-sphere. Furthermore, if denotes the class of the torus, and ''b'' denotes the class of the projective plane, then every element ''c'' of the monoid has a unique expression the form where is a positive integer and , or . We have . * Let \langle f\rangle be a cyclic monoid of order , that is, \langle f\rangle = \left\. Then f^n = f^k for some 0 \le k < n. In fact, each such gives a distinct monoid of order , and every cyclic monoid is isomorphic to one of these.
Moreover, can be considered as a function on the points \ given by :: \begin 0 & 1 & 2 & \cdots & n-2 & n-1 \\ 1 & 2 & 3 & \cdots & n-1 & k\end :or, equivalently :: f(i) := \begin i+1, & \text 0 \le i < n-1 \\ k, & \text i = n-1. \end :Multiplication of elements in \langle f\rangle is then given by function composition. :When k = 0 then the function is a permutation of \, and gives the unique cyclic group of order .


The monoid axioms imply that the identity element is unique: If and are identity elements of a monoid, then .

Products and powers

For each nonnegative integer , one can define the product p_n = \textstyle \prod_^n a_i of any sequence (a_1,\ldots,a_n) of elements of a monoid recursively: let and let for . As a special case, one can define nonnegative integer powers of an element of a monoid: and for . Then for all .

Invertible elements

An element is called inverse element, invertible if there exists an element such that and . The element is called the inverse of . Inverses, if they exist, are unique: If and are inverses of , then by associativity . If is invertible, say with inverse , then one can define negative powers of by setting for each ; this makes the equation hold for all . The set of all invertible elements in a monoid, together with the operation •, forms a group (mathematics), group.

Grothendieck group

Not every monoid sits inside a group. For instance, it is perfectly possible to have a monoid in which two elements and exist such that holds even though is not the identity element. Such a monoid cannot be embedded in a group, because in the group multiplying both sides with the inverse of would get that , which is not true. A monoid has the cancellation property (or is cancellative) if for all , and in , the equality implies , and the equality implies . A commutative monoid with the cancellation property can always be embedded in a group via the ''Grothendieck group construction''. That is how the additive group of the integers (a group with operation +) is constructed from the additive monoid of natural numbers (a commutative monoid with operation + and cancellation property). However, a non-commutative cancellative monoid need not be embeddable in a group. If a monoid has the cancellation property and is ''finite'', then it is in fact a group. The right- and left-cancellative elements of a monoid each in turn form a submonoid (i.e. are closed under the operation and obviously include the identity). This means that the cancellative elements of any commutative monoid can be extended to a group. The cancellative property in a monoid is not necessary to perform the Grothendieck construction – commutativity is sufficient. However, if a commutative monoid does not have the cancellation property, the homomorphism of the monoid into its Grothendieck group is not injective. More precisely, if , then and have the same image in the Grothendieck group, even if . In particular, if the monoid has an absorbing element, then its Grothendieck group is the trivial group.

Types of monoids

An inverse monoid is a monoid where for every ''a'' in ''M'', there exists a unique ''a''−1 in ''M'' such that and . If an inverse monoid is cancellative, then it is a group. In the opposite direction, a ''zerosumfree monoid'' is an additively written monoid in which implies that and : equivalently, that no element other than zero has an additive inverse.

Acts and operator monoids

Let ''M'' be a monoid, with the binary operation denoted by • and the identity element denoted by ''e''. Then a (left) ''M''-act (or left act over ''M'') is a set ''X'' together with an operation which is compatible with the monoid structure as follows: * for all ''x'' in ''X'': ; * for all ''a'', ''b'' in ''M'' and ''x'' in ''X'': . This is the analogue in monoid theory of a (left) Group action (mathematics), group action. Right ''M''-acts are defined in a similar way. A monoid with an act is also known as an ''operator monoid''. Important examples include transition systems of semiautomata. A transformation semigroup can be made into an operator monoid by adjoining the identity transformation.

Monoid homomorphisms

A homomorphism between two monoids and is a function such that * for all ''x'', ''y'' in ''M'' * , where ''e''''M'' and ''e''''N'' are the identities on ''M'' and ''N'' respectively. Monoid homomorphisms are sometimes simply called monoid morphisms. Not every semigroup homomorphism between monoids is a monoid homomorphism, since it may not map the identity to the identity of the target monoid, even though the identity is the identity of the image of homomorphism. For example, consider M_n, the set of residue classes modulo n equipped with multiplication. In particular, the class of 1 is the identity. Function f\colon M_3\to M_6 given by f(k)=3k is a semigroup homomorphism as 3k\cdot 3l = 9kl = 3kl in M_6. However, f(1)=3 \neq 1, so a monoid homomorphism is a semigroup homomorphism between monoids that maps the identity of the first monoid to the identity of the second monoid and the latter condition cannot be omitted. In contrast, a semigroup homomorphism between groups is always a group homomorphism, as it necessarily preserves the identity (because, in a group, the identity is the only element such that ). A bijective monoid homomorphism is called a monoid isomorphism. Two monoids are said to be isomorphic if there is a monoid isomorphism between them.

Equational presentation

Monoids may be given a ''presentation'', much in the same way that groups can be specified by means of a group presentation. One does this by specifying a set of generators Σ, and a set of relations on the free monoid Σ. One does this by extending (finite) binary relations on Σ to monoid congruences, and then constructing the quotient monoid, as above. Given a binary relation , one defines its symmetric closure as . This can be extended to a symmetric relation by defining if and only if and for some strings with . Finally, one takes the reflexive and transitive closure of ''E'', which is then a monoid congruence. In the typical situation, the relation ''R'' is simply given as a set of equations, so that R=\. Thus, for example, : \langle p,q\,\vert\; pq=1\rangle is the equational presentation for the bicyclic monoid, and : \langle a,b \,\vert\; aba=baa, bba=bab\rangle is the plactic monoid of degree 2 (it has infinite order). Elements of this plactic monoid may be written as a^ib^j(ba)^k for integers ''i'', ''j'', ''k'', as the relations show that ''ba'' commutes with both ''a'' and ''b''.

Relation to category theory

Monoids can be viewed as a special class of category theory, categories. Indeed, the axioms required of a monoid operation are exactly those required of morphism composition when restricted to the set of all morphisms whose source and target is a given object. That is, : ''A monoid is, essentially, the same thing as a category with a single object.'' More precisely, given a monoid , one can construct a small category with only one object and whose morphisms are the elements of ''M''. The composition of morphisms is given by the monoid operation •. Likewise, monoid homomorphisms are just functors between single object categories. So this construction gives an equivalence of categories, equivalence between the category of monoids, category of (small) monoids Mon and a full subcategory of the category of (small) categories Cat. Similarly, the category of groups is equivalent to another full subcategory of Cat. In this sense, category theory can be thought of as an extension of the concept of a monoid. Many definitions and theorems about monoids can be generalised to small categories with more than one object. For example, a quotient of a category with one object is just a quotient monoid. Monoids, just like other algebraic structures, also form their own category, Mon, whose objects are monoids and whose morphisms are monoid homomorphisms. There is also a notion of monoid (category theory), monoid object which is an abstract definition of what is a monoid in a category. A monoid object in category of sets, Set is just a monoid.

Monoids in computer science

In computer science, many abstract data types can be endowed with a monoid structure. In a common pattern, a sequence of elements of a monoid is "fold (higher-order function), folded" or "accumulated" to produce a final value. For instance, many iterative algorithms need to update some kind of "running total" at each iteration; this pattern may be elegantly expressed by a monoid operation. Alternatively, the associativity of monoid operations ensures that the operation can be parallelization, parallelized by employing a prefix sum or similar algorithm, in order to utilize multiple cores or processors efficiently. Given a sequence of values of type ''M'' with identity element \varepsilon and associative operation \bullet, the ''fold'' operation is defined as follows: : \mathrm: M^ \rarr M = \ell \mapsto \begin \varepsilon & \mbox \ell = \mathrm \\ m \bullet \mathrm \, \ell' & \mbox \ell = \mathrm \, m \, \ell' \end In addition, any data structure can be 'folded' in a similar way, given a serialization of its elements. For instance, the result of "folding" a binary tree might differ depending on pre-order vs. post-order tree traversal.


An application of monoids in computer science is so-called MapReduce programming model (se
Encoding Map-Reduce As A Monoid With Left Folding
. MapReduce, in computing, consists of two or three operations. Given a dataset, "Map" consists of mapping arbitrary data to elements of a specific monoid. "Reduce" consists of folding those elements, so that in the end we produce just one element. For example, if we have a multiset, in a program it is represented as a map from elements to their numbers. Elements are called keys in this case. The number of distinct keys may be too big, and in this case the multiset is being sharded. To finalize reduction properly, the "Shuffling" stage regroups the data among the nodes. If we do not need this step, the whole Map/Reduce consists of mapping and reducing; both operation are parallelizable, the former due to its element-wise nature, the latter due to associativity of the monoid.

Complete monoids

A complete monoid is a commutative monoid equipped with an Finitary, infinitary sum operation \Sigma_I for any index set such that:Droste, M., & Kuich, W. (2009). Semirings and Formal Power Series. ''Handbook of Weighted Automata'', 3–28. , pp. 7–10 : \sum_ =0;\quad \sum_ = m_j;\quad \sum_ = m_j+m_k \quad \text j\neq k and : \sum_ = \sum_(m_i)\quad \text \bigcup_ I_j=I \text I_j \cap I_ = \emptyset \quad \text j\neq j' A continuous monoid is an ordered commutative monoid in which every directed set has a least upper bound compatible with the monoid operation: : a + \sup S = \sup(a + S) \ . These two concepts are closely related: a continuous monoid is a complete monoid in which the infinitary sum may be defined as : \sum_I a_i = \sup \sum_E a_i where the supremum on the right runs over all finite subsets of and each sum on the right is a finite sum in the monoid.

See also

* Green's relations * Monad (functional programming) * Semiring and Kleene algebra * Star height problem * Vedic square



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External links

* * * {{PlanetMath, urlname=Monoid , title=Monoid , id=389 Algebraic structures Category theory Semigroup theory