In mathematics, a **group** is a set equipped with an operation that combines any two elements to form a third element while being associative as well as having an identity element and inverse elements. These three conditions, called group axioms, hold for number systems and many other mathematical structures. For example, the integers together with the addition operation form a group. The formulation of the axioms is, however, detached from the concrete nature of the group and its operation. This allows one to handle entities of very different mathematical origins in a flexible way, while retaining essential structural aspects of many objects in abstract algebra and beyond. The ubiquity of groups in numerous areas—both within and outside mathematics—makes them a central organizing principle of contemporary mathematics.^{ [1] }^{ [2] }

- Definition and illustration
- First example: the integers
- Definition
- Notation and terminology
- Second example: a symmetry group
- History
- Elementary consequences of the group axioms
- Uniqueness of identity element
- Uniqueness of inverses
- Division
- Basic concepts
- Group homomorphisms
- Subgroups
- Cosets
- Quotient groups
- Examples and applications
- Numbers
- Modular arithmetic
- Cyclic groups
- Symmetry groups
- General linear group and representation theory
- Galois groups
- Finite groups
- Classification of finite simple groups
- Groups with additional structure
- Topological groups
- Lie groups
- Generalizations
- See also
- Notes
- Citations
- References
- General references
- Special references
- Historical references
- External links

Groups share a fundamental kinship with the notion of symmetry. For example, a symmetry group encodes symmetry features of a geometrical object: the group consists of the set of transformations that leave the object unchanged and the operation of combining two such transformations by performing one after the other. Lie groups arise as symmetry groups in geometry but appear also in the Standard Model of particle physics. The Poincaré group is a Lie group consisting of the symmetries of spacetime in special relativity. Point groups describe symmetry in molecular chemistry.

The concept of a group arose from the study of polynomial equations, starting with Évariste Galois in the 1830s, who introduced the term of *group* (*groupe*, in French) for the symmetry group of the roots of an equation, now called a Galois group. After contributions from other fields such as number theory and geometry, the group notion was generalized and firmly established around 1870. Modern group theory—an active mathematical discipline—studies groups in their own right. To explore groups, mathematicians have devised various notions to break groups into smaller, better-understandable pieces, such as subgroups, quotient groups and simple groups. In addition to their abstract properties, group theorists also study the different ways in which a group can be expressed concretely, both from a point of view of representation theory (that is, through the representations of the group) and of computational group theory. A theory has been developed for finite groups, which culminated with the classification of finite simple groups, completed in 2004. Since the mid-1980s, geometric group theory, which studies finitely generated groups as geometric objects, has become an active area in group theory.

Algebraic structure → Group theoryGroup theory |
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One of the more familiar groups is the set of integers

together with addition.^{ [3] } For any two integers and , the sum is also an integer; this * closure * property says that is a binary operation on . The following properties of integer addition serve as a model for the group axioms in the definition below.

- For all integers , and , one has . Expressed in words, adding to first, and then adding the result to gives the same final result as adding to the sum of and . This property is known as
*associativity*. - If is any integer, then and . Zero is called the
*identity element*of addition because adding it to any integer returns the same integer. - For every integer , there is an integer such that and . The integer is called the
*inverse element*of the integer and is denoted .

The integers, together with the operation , form a mathematical object belonging to a broad class sharing similar structural aspects. To appropriately understand these structures as a collective, the following definition is developed.

The axioms for a group are short and natural... Yet somehow hidden behind these axioms is the monster simple group, a huge and extraordinary mathematical object, which appears to rely on numerous bizarre coincidences to exist. The axioms for groups give no obvious hint that anything like this exists.

Richard Borcherds in *Mathematicians: An Outer View of the Inner World*^{ [4] }

A group is a set together with a binary operation on , here denoted "", that combines any two elements and to form an element of , denoted , such that the following three requirements, known as *group axioms*, are satisfied:^{ [5] }^{ [6] }^{ [7] }^{ [lower-alpha 1] }

- Associativity
- For all , , in , one has .
- Identity element
- There exists an element in such that, for every in , one has and .
- Such an element is unique (see below). It is called
*the identity element*of the group. - Inverse element
- For each in , there exists an element in such that and , where is the identity element.
- For each , the element is unique (see below); it is called
*the inverse*of and is commonly denoted .

Formally, the group is the ordered pair of a set and a binary operation on this set that satisfies the group axioms. The set is called the *underlying set* of the group, and the operation is called the *group operation* or the *group law*.

A group and its underlying set are thus two different mathematical objects. To avoid cumbersome notation, it is common to abuse notation by using the same symbol to denote both. This reflects also an informal way of thinking: that the group is the same as the set except that it has been enriched by additional structure provided by the operation.

For example, consider the set of real numbers , which has the operations of addition and multiplication . Formally, is a set, is a group, and is a field. But it is common to write to denote any of these three objects.

The *additive group* of the field is the group whose underlying set is and whose operation is addition. The *multiplicative group* of the field is the group whose underlying set is the set of nonzero real numbers and whose operation is multiplication.

More generally, one speaks of an *additive group* whenever the group operation is notated as addition; in this case, the identity is typically denoted , and the inverse of an element is denoted . Similarly, one speaks of a *multiplicative group* whenever the group operation is notated as multiplication; in this case, the identity is typically denoted , and the inverse of an element is denoted . In a multiplicative group, the operation symbol is usually omitted entirely, so that the operation is denoted by juxtaposition, instead of .

The definition of a group does not require that for all elements and in . If this additional condition holds, then the operation is said to be commutative, and the group is called an abelian group. It is a common convention that for an abelian group either additive or multiplicative notation may be used, but for a nonabelian group only multiplicative notation is used.

Several other notations are commonly used for groups whose elements are not numbers. For a group whose elements are functions, the operation is often function composition ; then the identity may be denoted id. In the more specific cases of geometric transformation groups, symmetry groups, permutation groups, and automorphism groups, the symbol is often omitted, as for multiplicative groups. Many other variants of notation may be encountered.

Two figures in the plane are congruent if one can be changed into the other using a combination of rotations, reflections, and translations. Any figure is congruent to itself. However, some figures are congruent to themselves in more than one way, and these extra congruences are called symmetries. A square has eight symmetries. These are:

- the identity operation leaving everything unchanged, denoted id;
- rotations of the square around its center by 90°, 180°, and 270° clockwise, denoted by , and , respectively;
- reflections about the horizontal and vertical middle line ( and ), or through the two diagonals ( and ).

These symmetries are functions. Each sends a point in the square to the corresponding point under the symmetry. For example, sends a point to its rotation 90° clockwise around the square's center, and sends a point to its reflection across the square's vertical middle line. Composing two of these symmetries gives another symmetry. These symmetries determine a group called the dihedral group of degree four, denoted . The underlying set of the group is the above set of symmetries, and the group operation is function composition.^{ [8] } Two symmetries are combined by composing them as functions, that is, applying the first one to the square, and the second one to the result of the first application. The result of performing first and then is written symbolically *from right to left* as ("apply the symmetry after performing the symmetry "). This is the usual notation for composition of functions.

The group table lists the results of all such compositions possible. For example, rotating by 270° clockwise () and then reflecting horizontally () is the same as performing a reflection along the diagonal (). Using the above symbols, highlighted in blue in the group table:

The elements , , , and form a subgroup whose group table is highlighted in red (upper left region). A left and right coset of this subgroup is highlighted in green (in the last row) and yellow (last column), respectively. |

Given this set of symmetries and the described operation, the group axioms can be understood as follows.

*Binary operation*: Composition is a binary operation. That is, is a symmetry for any two symmetries and . For example,

that is, rotating 270° clockwise after reflecting horizontally equals reflecting along the counter-diagonal (). Indeed every other combination of two symmetries still gives a symmetry, as can be checked using the group table.

*Associativity*: The associativity axiom deals with composing more than two symmetries: Starting with three elements , and of , there are two possible ways of using these three symmetries in this order to determine a symmetry of the square. One of these ways is to first compose and into a single symmetry, then to compose that symmetry with . The other way is to first compose and , then to compose the resulting symmetry with . These two ways must give always the same result, that is,

For example, can be checked using the group table:

*Identity element*: The identity element is , as it does not change any symmetry when composed with it either on the left or on the right.

*Inverse element*: Each symmetry has an inverse: , the reflections , , , and the 180° rotation are their own inverse, because performing them twice brings the square back to its original orientation. The rotations and are each other's inverses, because rotating 90° and then rotation 270° (or vice versa) yields a rotation over 360° which leaves the square unchanged. This is easily verified on the table.

In contrast to the group of integers above, where the order of the operation is irrelevant, it does matter in , as, for example, but . In other words, is not abelian.

The modern concept of an abstract group developed out of several fields of mathematics.^{ [9] }^{ [10] }^{ [11] } The original motivation for group theory was the quest for solutions of polynomial equations of degree higher than 4. The 19th-century French mathematician Évariste Galois, extending prior work of Paolo Ruffini and Joseph-Louis Lagrange, gave a criterion for the solvability of a particular polynomial equation in terms of the symmetry group of its roots (solutions). The elements of such a Galois group correspond to certain permutations of the roots. At first, Galois' ideas were rejected by his contemporaries, and published only posthumously.^{ [12] }^{ [13] } More general permutation groups were investigated in particular by Augustin Louis Cauchy. Arthur Cayley's *On the theory of groups, as depending on the symbolic equation * (1854) gives the first abstract definition of a finite group.^{ [14] }

Geometry was a second field in which groups were used systematically, especially symmetry groups as part of Felix Klein's 1872 Erlangen program.^{ [15] } After novel geometries such as hyperbolic and projective geometry had emerged, Klein used group theory to organize them in a more coherent way. Further advancing these ideas, Sophus Lie founded the study of Lie groups in 1884.^{ [16] }

The third field contributing to group theory was number theory. Certain abelian group structures had been used implicitly in Carl Friedrich Gauss' number-theoretical work * Disquisitiones Arithmeticae * (1798), and more explicitly by Leopold Kronecker.^{ [17] } In 1847, Ernst Kummer made early attempts to prove Fermat's Last Theorem by developing groups describing factorization into prime numbers.^{ [18] }

The convergence of these various sources into a uniform theory of groups started with Camille Jordan's *Traité des substitutions et des équations algébriques* (1870).^{ [19] } Walther von Dyck (1882) introduced the idea of specifying a group by means of generators and relations, and was also the first to give an axiomatic definition of an "abstract group", in the terminology of the time.^{ [20] } As of the 20th century, groups gained wide recognition by the pioneering work of Ferdinand Georg Frobenius and William Burnside, who worked on representation theory of finite groups, Richard Brauer's modular representation theory and Issai Schur's papers.^{ [21] } The theory of Lie groups, and more generally locally compact groups was studied by Hermann Weyl, Élie Cartan and many others.^{ [22] } Its algebraic counterpart, the theory of algebraic groups, was first shaped by Claude Chevalley (from the late 1930s) and later by the work of Armand Borel and Jacques Tits.^{ [23] }

The University of Chicago's 1960–61 Group Theory Year brought together group theorists such as Daniel Gorenstein, John G. Thompson and Walter Feit, laying the foundation of a collaboration that, with input from numerous other mathematicians, led to the classification of finite simple groups, with the final step taken by Aschbacher and Smith in 2004. This project exceeded previous mathematical endeavours by its sheer size, in both length of proof and number of researchers. Research concerning this classification proof is ongoing.^{ [24] } These days, group theory is still a highly active mathematical branch,^{ [lower-alpha 2] } impacting many other fields, as the examples below illustrate.

Basic facts about all groups that can be obtained directly from the group axioms are commonly subsumed under *elementary group theory*.^{ [25] } For example, repeated applications of the associativity axiom show that the unambiguity of

generalizes to more than three factors. Because this implies that parentheses can be inserted anywhere within such a series of terms, parentheses are usually omitted.^{ [26] }

Individual axioms may be "weakened" to assert only the existence of a left identity and left inverses. From these *one-sided axioms*, one can prove that the left identity is also a right identity and a left inverse is also a right inverse for the same element. Since they define exactly the same structures as groups, collectively the axioms are no weaker.^{ [27] }

The group axioms imply that the identity element is unique: If and are identity elements of a group, then . Therefore it is customary to speak of *the* identity.^{ [28] }

The group axioms also imply that the inverse of each element is unique: If a group element has both and as inverses, then

since is the identity element | ||||

since is an inverse of , so | ||||

by associativity, which allows rearranging the parentheses | ||||

since is an inverse of , so | ||||

since is the identity element. |

Therefore it is customary to speak of *the* inverse of an element.^{ [28] }

Given elements and of a group , there is a unique solution in to the equation , namely . (One usually avoids using fraction notation unless is abelian, because of the ambiguity of whether it means or .)^{ [29] } It follows that for each in , the function that maps each to is a bijection; it is called *left multiplication by * or *left translation by *.

Similarly, given and , the unique solution to is . For each , the function that maps each to is a bijection called *right multiplication by * or *right translation by *.

When studying sets, one uses concepts such as subset, function, and quotient by an equivalence relation. When studying groups, one uses instead subgroups, homomorphisms, and quotient groups. These are the appropriate analogues that take into account the existence of the group structure.^{ [lower-alpha 3] }

Group homomorphisms^{ [lower-alpha 4] } are functions that respect group structure; they may be used to relate two groups. A *homomorphism* from a group to a group is a function such that

for all elements and in .

It would be natural to require also that respect identities, , and inverses, for all in . However, these additional requirements need not be included in the definition of homomorphisms, because they are already implied by the requirement of respecting the group operation.^{ [30] }

The *identity homomorphism* of a group is the homomorphism that maps each element of to itself. An *inverse homomorphism* of a homomorphism is a homomorphism such that and , that is, such that for all in and such that for all in . An * isomorphism * is a homomorphism that has an inverse homomorphism; equivalently, it is a bijective homomorphism. Groups and are called *isomorphic* if there exists an isomorphism . In this case, can be obtained from simply by renaming its elements according to the function ; then any statement true for is true for , provided that any specific elements mentioned in the statement are also renamed.

The collection of all groups, together with the homomorphisms between them, form a category, the category of groups.^{ [31] }

Informally, a *subgroup* is a group contained within a bigger one, : it has a subset of the elements of , with the same operation.^{ [32] } Concretely, this means that the identity element of must be contained in , and whenever and are both in , then so are and , so the elements of , equipped with the group operation on restricted to , indeed form a group.

In the example of symmetries of a square, the identity and the rotations constitute a subgroup , highlighted in red in the group table of the example: any two rotations composed are still a rotation, and a rotation can be undone by (i.e., is inverse to) the complementary rotations 270° for 90°, 180° for 180°, and 90° for 270°. The subgroup test provides a necessary and sufficient condition for a nonempty subset *H* of a group *G* to be a subgroup: it is sufficient to check that for all elements and in . Knowing a group's subgroups is important in understanding the group as a whole.^{ [lower-alpha 5] }

Given any subset of a group , the subgroup generated by consists of products of elements of and their inverses. It is the smallest subgroup of containing .^{ [33] } In the example of symmetries of a square, the subgroup generated by and consists of these two elements, the identity element , and the element . Again, this is a subgroup, because combining any two of these four elements or their inverses (which are, in this particular case, these same elements) yields an element of this subgroup.

In many situations it is desirable to consider two group elements the same if they differ by an element of a given subgroup. For example, in the symmetry group of a square, once any reflection is performed, rotations alone cannot return the square to its original position, so one can think of the reflected positions of the square as all being equivalent to each other, and as inequivalent to the unreflected positions; the rotation operations are irrelevant to the question whether a reflection has been performed. Cosets are used to formalize this insight: a subgroup determines left and right cosets, which can be thought of as translations of by an arbitrary group element . In symbolic terms, the *left* and *right* cosets of , containing an element , are

and , respectively.^{ [34] }

The left cosets of any subgroup form a partition of ; that is, the union of all left cosets is equal to and two left cosets are either equal or have an empty intersection.^{ [35] } The first case happens precisely when , i.e., when the two elements differ by an element of . Similar considerations apply to the right cosets of . The left cosets of may or may not be the same as its right cosets. If they are (that is, if all in satisfy ), then is said to be a * normal subgroup *.

In , the group of symmetries of a square, with its subgroup of rotations, the left cosets are either equal to , if is an element of itself, or otherwise equal to (highlighted in green in the group table of ). The subgroup is normal, because and similarly for the other elements of the group. (In fact, in the case of , the cosets generated by reflections are all equal: .)

In some situations the set of cosets of a subgroup can be endowed with a group law, giving a *quotient group* or *factor group*. For this to be possible, the subgroup has to be normal. Given any normal subgroup *N*, the quotient group is defined by

where the notation is read as " modulo ".^{ [36] }}} This set inherits a group operation (sometimes called coset multiplication, or coset addition) from the original group : the product of two cosets and is for all and in . This definition is motivated by the idea (itself an instance of general structural considerations outlined above) that the map that associates to any element its coset should be a group homomorphism, or by general abstract considerations called universal properties. The coset serves as the identity in this group, and the inverse of in the quotient group is .^{ [lower-alpha 6] }

The elements of the quotient group are itself, which represents the identity, and . The group operation on the quotient is shown in the table. For example, . Both the subgroup , as well as the corresponding quotient are abelian, whereas is not abelian. Building bigger groups by smaller ones, such as from its subgroup and the quotient is abstracted by a notion called semidirect product.

Quotient groups and subgroups together form a way of describing every group by its * presentation *: any group is the quotient of the free group over the * generators * of the group, quotiented by the subgroup of *relations*. The dihedral group , for example, can be generated by two elements and (for example, , the right rotation and the vertical (or any other) reflection), which means that every symmetry of the square is a finite composition of these two symmetries or their inverses. Together with the relations

the group is completely described.^{ [37] } A presentation of a group can also be used to construct the Cayley graph, a device used to graphically capture discrete groups.^{ [38] }

Sub- and quotient groups are related in the following way: a subgroup of corresponds to an injective map , for which any element of the target has at most one element that maps to it. The counterpart to injective maps are surjective maps (every element of the target is mapped onto), such as the canonical map .^{ [lower-alpha 7] } Interpreting subgroup and quotients in light of these homomorphisms emphasizes the structural concept inherent to these definitions. In general, homomorphisms are neither injective nor surjective. The kernel and image of group homomorphisms and the first isomorphism theorem address this phenomenon.

Examples and applications of groups abound. A starting point is the group of integers with addition as group operation, introduced above. If instead of addition multiplication is considered, one obtains multiplicative groups. These groups are predecessors of important constructions in abstract algebra.

Groups are also applied in many other mathematical areas. Mathematical objects are often examined by associating groups to them and studying the properties of the corresponding groups. For example, Henri Poincaré founded what is now called algebraic topology by introducing the fundamental group.^{ [39] } By means of this connection, topological properties such as proximity and continuity translate into properties of groups.^{ [lower-alpha 8] } For example, elements of the fundamental group are represented by loops. The second image shows some loops in a plane minus a point. The blue loop is considered null-homotopic (and thus irrelevant), because it can be continuously shrunk to a point. The presence of the hole prevents the orange loop from being shrunk to a point. The fundamental group of the plane with a point deleted turns out to be infinite cyclic, generated by the orange loop (or any other loop winding once around the hole). This way, the fundamental group detects the hole.

In more recent applications, the influence has also been reversed to motivate geometric constructions by a group-theoretical background.^{ [lower-alpha 9] } In a similar vein, geometric group theory employs geometric concepts, for example in the study of hyperbolic groups.^{ [40] } Further branches crucially applying groups include algebraic geometry and number theory.^{ [41] }

In addition to the above theoretical applications, many practical applications of groups exist. Cryptography relies on the combination of the abstract group theory approach together with algorithmical knowledge obtained in computational group theory, in particular when implemented for finite groups.^{ [42] } Applications of group theory are not restricted to mathematics; sciences such as physics, chemistry and computer science benefit from the concept.

Many number systems, such as the integers and the rationals enjoy a naturally given group structure. In some cases, such as with the rationals, both addition and multiplication operations give rise to group structures. Such number systems are predecessors to more general algebraic structures known as rings and fields. Further abstract algebraic concepts such as modules, vector spaces and algebras also form groups.

The group of integers under addition, denoted , has been described above. The integers, with the operation of multiplication instead of addition, do *not* form a group. The associativity and identity axioms are satisfied, but inverses do not exist: for example, is an integer, but the only solution to the equation in this case is , which is a rational number, but not an integer. Hence not every element of has a (multiplicative) inverse.^{ [lower-alpha 10] }

The desire for the existence of multiplicative inverses suggests considering fractions

Fractions of integers (with nonzero) are known as rational numbers.^{ [lower-alpha 11] } The set of all such irreducible fractions is commonly denoted . There is still a minor obstacle for , the rationals with multiplication, being a group: because zero does not have a multiplicative inverse (i.e., there is no such that ), is still not a group.

However, the set of all *nonzero* rational numbers does form an abelian group under multiplication, also denoted .^{ [lower-alpha 12] } Associativity and identity element axioms follow from the properties of integers. The closure requirement still holds true after removing zero, because the product of two nonzero rationals is never zero. Finally, the inverse of is , therefore the axiom of the inverse element is satisfied.

The rational numbers (including zero) also form a group under addition. Intertwining addition and multiplication operations yields more complicated structures called rings and – if division by other than zero is possible, such as in – fields, which occupy a central position in abstract algebra. Group theoretic arguments therefore underlie parts of the theory of those entities.^{ [lower-alpha 13] }

Modular arithmetic for a *modulus* defines any two elements and that differ by a multiple of to be equivalent, denoted by . Every integer is equivalent to one of the integers from to , and the operations of modular arithmetic modify normal arithmetic by replacing the result of any operation by its equivalent representative. Modular addition, defined in this way for the integers from to , forms a group, denoted as or , with as the identity element and as the inverse element of .

A familiar example is addition of hours on the face of a clock, where 12 rather than 0 is chosen as the representative of the identity. If the hour hand is on and is advanced hours, it ends up on , as shown in the illustration. This is expressed by saying that is congruent to "modulo " or, in symbols,

For any prime number , there is also the multiplicative group of integers modulo .^{ [43] } Its elements can be represented by to . The group operation, multiplication modulo , replaces the usual product by its representative, the remainder of division by . For example, for , the four group elements can be represented by . In this group, , because the usual product is equivalent to : when divided by it yields a remainder of . The primality of ensures that the usual product of two representatives is not divisible by , and therefore that the modular product is nonzero.^{ [lower-alpha 14] } The identity element is represented by , and associativity follows from the corresponding property of the integers. Finally, the inverse element axiom requires that given an integer not divisible by , there exists an integer such that

that is, such that evenly divides . The inverse can be found by using Bézout's identity and the fact that the greatest common divisor equals .^{ [44] } In the case above, the inverse of the element represented by is that represented by , and the inverse of the element represented by is represented by , as . Hence all group axioms are fulfilled. This example is similar to above: it consists of exactly those elements in the ring that have a multiplicative inverse.^{ [45] } These groups, denoted , are crucial to public-key cryptography.^{ [lower-alpha 15] }

A *cyclic group* is a group all of whose elements are powers of a particular element .^{ [46] } In multiplicative notation, the elements of the group are

where means , stands for , etc.^{ [lower-alpha 16] } Such an element is called a generator or a primitive element of the group. In additive notation, the requirement for an element to be primitive is that each element of the group can be written as

In the groups introduced above, the element is primitive, so these groups are cyclic. Indeed, each element is expressible as a sum all of whose terms are . Any cyclic group with elements is isomorphic to this group. A second example for cyclic groups is the group of th complex roots of unity, given by complex numbers satisfying . These numbers can be visualized as the vertices on a regular -gon, as shown in blue in the image for . The group operation is multiplication of complex numbers. In the picture, multiplying with corresponds to a counter-clockwise rotation by 60°.^{ [47] } From field theory, the group is cyclic for prime : for example, if , is a generator since , , , and .

Some cyclic groups have an infinite number of elements. In these groups, for every non-zero element , all the powers of are distinct; despite the name "cyclic group", the powers of the elements do not cycle. An infinite cyclic group is isomorphic to , the group of integers under addition introduced above.^{ [48] } As these two prototypes are both abelian, so are all cyclic groups.

The study of finitely generated abelian groups is quite mature, including the fundamental theorem of finitely generated abelian groups; and reflecting this state of affairs, many group-related notions, such as center and commutator, describe the extent to which a given group is not abelian.^{ [49] }

*Symmetry groups* are groups consisting of symmetries of given mathematical objects, principally geometric entities, such as the symmetry group of the square given as an introductory example above, although they also arise in algebra such as the symmetries among the roots of polynomial equations dealt with in Galois theory (see below).^{ [50] } Conceptually, group theory can be thought of as the study of symmetry.^{ [lower-alpha 17] } Symmetries in mathematics greatly simplify the study of geometrical or analytical objects. A group is said to act on another mathematical object *X* if every group element can be associated to some operation on *X* and the composition of these operations follows the group law. For example, an element of the (2,3,7) triangle group acts on a triangular tiling of the hyperbolic plane by permuting the triangles.^{ [51] } By a group action, the group pattern is connected to the structure of the object being acted on.

In chemical fields, such as crystallography, space groups and point groups describe molecular symmetries and crystal symmetries. These symmetries underlie the chemical and physical behavior of these systems, and group theory enables simplification of quantum mechanical analysis of these properties.^{ [52] } For example, group theory is used to show that optical transitions between certain quantum levels cannot occur simply because of the symmetry of the states involved.^{ [53] }

Not only are groups useful to assess the implications of symmetries in molecules, but surprisingly they also predict that molecules sometimes can change symmetry. The Jahn–Teller effect is a distortion of a molecule of high symmetry when it adopts a particular ground state of lower symmetry from a set of possible ground states that are related to each other by the symmetry operations of the molecule.^{ [54] }^{ [55] }

Likewise, group theory helps predict the changes in physical properties that occur when a material undergoes a phase transition, for example, from a cubic to a tetrahedral crystalline form. An example is ferroelectric materials, where the change from a paraelectric to a ferroelectric state occurs at the Curie temperature and is related to a change from the high-symmetry paraelectric state to the lower symmetry ferroelectric state, accompanied by a so-called soft phonon mode, a vibrational lattice mode that goes to zero frequency at the transition.^{ [56] }

Such spontaneous symmetry breaking has found further application in elementary particle physics, where its occurrence is related to the appearance of Goldstone bosons.^{ [57] }

Buckminsterfullerene displays icosahedral symmetry ^{ [58] } | Ammonia, NH_{3}. Its symmetry group is of order 6, generated by a 120° rotation and a reflection.^{ [59] } | Cubane C_{8}H_{8} featuresoctahedral symmetry. ^{ [60] } | Hexaaquacopper(II) complex ion, [Cu (OH_{2})_{6}]^{2+} is distorted from a perfectly symmetric shape because of the Jahn–Teller effect.^{ [61] } | The (2,3,7) triangle group, a hyperbolic group, acts on this tiling of the hyperbolic plane.^{ [51] } |

Finite symmetry groups such as the Mathieu groups are used in coding theory, which is in turn applied in error correction of transmitted data, and in CD players.^{ [62] } Another application is differential Galois theory, which characterizes functions having antiderivatives of a prescribed form, giving group-theoretic criteria for when solutions of certain differential equations are well-behaved.^{ [lower-alpha 18] } Geometric properties that remain stable under group actions are investigated in (geometric) invariant theory.^{ [63] }

Matrix groups consist of matrices together with matrix multiplication. The *general linear group* consists of all invertible -by- matrices with real entries.^{ [64] } Its subgroups are referred to as *matrix groups* or * linear groups *. The dihedral group example mentioned above can be viewed as a (very small) matrix group. Another important matrix group is the special orthogonal group . It describes all possible rotations in dimensions. Rotation matrices in this group are used in computer graphics.^{ [65] }

*Representation theory* is both an application of the group concept and important for a deeper understanding of groups.^{ [66] }^{ [67] } It studies the group by its group actions on other spaces. A broad class of group representations are linear representations in which the group acts on a vector space, such as the three-dimensional Euclidean space . A representation of a group on an -dimensional real vector space is simply a group homomorphism from the group to the general linear group. This way, the group operation, which may be abstractly given, translates to the multiplication of matrices making it accessible to explicit computations.^{ [lower-alpha 19] }

A group action gives further means to study the object being acted on.^{ [lower-alpha 20] } On the other hand, it also yields information about the group. Group representations are an organizing principle in the theory of finite groups, Lie groups, algebraic groups and topological groups, especially (locally) compact groups.^{ [66] }^{ [68] }

*Galois groups* were developed to help solve polynomial equations by capturing their symmetry features.^{ [69] }^{ [70] } For example, the solutions of the quadratic equation are given by

Each solution can be obtained by replacing the sign by or ; analogous formula are known for cubic and quartic equations, but do *not* exist in general for degree 5 and higher.^{ [71] } In the quadratic formula, changing the sign (permuting the resulting two solutions) can be viewed as a (very simple) group operation. Analogous Galois groups act on the solutions of higher-degree polynomials and are closely related to the existence of formulas for their solution. Abstract properties of these groups (in particular their solvability) give a criterion for the ability to express the solutions of these polynomials using solely addition, multiplication, and roots similar to the formula above.^{ [72] }

Modern Galois theory generalizes the above type of Galois groups by shifting to field theory and considering field extensions formed as the splitting field of a polynomial. This theory establishes—via the fundamental theorem of Galois theory—a precise relationship between fields and groups, underlining once again the ubiquity of groups in mathematics.^{ [73] }

A group is called *finite* if it has a finite number of elements. The number of elements is called the order of the group.^{ [74] } An important class is the * symmetric groups *, the groups of permutations of objects. For example, the symmetric group on 3 letters is the group of all possible reorderings of the objects. The three letters ABC can be reordered into ABC, ACB, BAC, BCA, CAB, CBA, forming in total 6 (factorial of 3) elements. The group operation is composition of these reorderings, and the identity element is the reordering operation that leaves the order unchanged. This class is fundamental insofar as any finite group can be expressed as a subgroup of a symmetric group for a suitable integer , according to Cayley's theorem. Parallel to the group of symmetries of the square above, can also be interpreted as the group of symmetries of an equilateral triangle.

The order of an element in a group is the least positive integer such that , where represents

that is, application of the operation "" to copies of . (If "" represents multiplication, then corresponds to the th power of .) In infinite groups, such an may not exist, in which case the order of is said to be infinity. The order of an element equals the order of the cyclic subgroup generated by this element.

More sophisticated counting techniques, for example, counting cosets, yield more precise statements about finite groups: Lagrange's Theorem states that for a finite group the order of any finite subgroup divides the order of . The Sylow theorems give a partial converse.

The dihedral group of symmetries of a square is a finite group of order 8. In this group, the order of is 4, as is the order of the subgroup that this element generates. The order of the reflection elements etc. is 2. Both orders divide 8, as predicted by Lagrange's theorem. The groups of multiplication modulo a prime have order .

Mathematicians often strive for a complete classification (or list) of a mathematical notion. In the context of finite groups, this aim leads to difficult mathematics. According to Lagrange's theorem, finite groups of order , a prime number, are necessarily cyclic groups and hence also abelian. Groups of order can also be shown to be abelian, a statement which does not generalize to order , as the non-abelian group of order above shows.^{ [75] } Computer algebra systems can be used to list small groups, but there is no classification of all finite groups.^{ [lower-alpha 21] } An intermediate step is the classification of finite simple groups.^{ [lower-alpha 22] } A nontrivial group is called * simple * if its only normal subgroups are the trivial group and the group itself.^{ [lower-alpha 23] } The Jordan–Hölder theorem exhibits finite simple groups as the building blocks for all finite groups.^{ [76] } Listing all finite simple groups was a major achievement in contemporary group theory. 1998 Fields Medal winner Richard Borcherds succeeded in proving the monstrous moonshine conjectures, a surprising and deep relation between the largest finite simple sporadic group (the "monster group") and certain modular functions, a piece of classical complex analysis, and string theory, a theory supposed to unify the description of many physical phenomena.^{ [77] }

An equivalent definition of group consists of replacing the "there exist" part of the group axioms by operations whose result is the element that must exist. So, a group is a set equipped with a binary operation (the group operation), a unary operation (which provides the inverse) and a nullary operation, which has no operand and results in the identity element. Otherwise, the group axioms are exactly the same. This variant of the definition avoids existential quantifiers and is used in computing with groups and for computer-aided proofs.

This way of defining groups lends itself to generalizations such as the notion of a group objects in a category. Briefly this is an object (that is, examples of another mathematical structure) which comes with transformations (called morphisms) that mimic the group axioms.^{ [78] }

Some topological spaces may be endowed with a group law. In order for the group law and the topology to interweave well, the group operations must be continuous functions; informally, and must not vary wildly if and vary only a little. Such groups are called *topological groups,* and they are the group objects in the category of topological spaces.^{ [79] } The most basic examples are the group of real numbers under addition and the group of nonzero real numbers under multiplication. Similar examples can be formed from any other topological field, such as the field of complex numbers or the field of *p*-adic numbers. These examples are locally compact, so they have Haar measures and can be studied via harmonic analysis. Other locally compact topological groups include the group of points of an algebraic group over a local field or adele ring; these are basic to number theory^{ [80] } Galois groups of infinite algebraic field extensions are equipped with the Krull topology, which plays a role in infinite Galois theory.^{ [81] } A generalization used in algebraic geometry is the étale fundamental group.^{ [82] }

A *Lie group* is a group that also has the structure of a differentiable manifold; informally, this means that it looks locally like a Euclidean space of some fixed dimension.^{ [83] } Again, the definition requires the additional structure, here the manifold structure, to be compatible: the multiplication and inverse maps are required to be smooth.

A standard example is the general linear group introduced above: it is an open subset of the space of all -by- matrices, because it is given by the inequality

where denotes an -by- matrix.^{ [84] }

Lie groups are of fundamental importance in modern physics: Noether's theorem links continuous symmetries to conserved quantities.^{ [85] } Rotation, as well as translations in space and time are basic symmetries of the laws of mechanics. They can, for instance, be used to construct simple models—imposing, say, axial symmetry on a situation will typically lead to significant simplification in the equations one needs to solve to provide a physical description.^{ [lower-alpha 24] } Another example is the group of Lorentz transformations, which relate measurements of time and velocity of two observers in motion relative to each other. They can be deduced in a purely group-theoretical way, by expressing the transformations as a rotational symmetry of Minkowski space. The latter serves—in the absence of significant gravitation—as a model of spacetime in special relativity.^{ [86] } The full symmetry group of Minkowski space, i.e., including translations, is known as the Poincaré group. By the above, it plays a pivotal role in special relativity and, by implication, for quantum field theories.^{ [87] } Symmetries that vary with location are central to the modern description of physical interactions with the help of gauge theory. An important example of a gauge theory is the Standard Model, which describes three of the four known fundamental forces and classifies all known elementary particles.^{ [88] }

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. |

In abstract algebra, more general structures are defined by relaxing some of the axioms defining a group.^{ [31] }^{ [89] }^{ [90] } For example, if the requirement that every element has an inverse is eliminated, the resulting algebraic structure is called a monoid. The natural numbers (including zero) under addition form a monoid, as do the nonzero integers under multiplication , see above. There is a general method to formally add inverses to elements to any (abelian) monoid, much the same way as is derived from , known as the Grothendieck group. Groupoids are similar to groups except that the composition need not be defined for all and . They arise in the study of more complicated forms of symmetry, often in topological and analytical structures, such as the fundamental groupoid or stacks. Finally, it is possible to generalize any of these concepts by replacing the binary operation with an arbitrary n-ary one (i.e., an operation taking n arguments). With the proper generalization of the group axioms this gives rise to an n-ary group ^{ [91] } The table gives a list of several structures generalizing groups.

- ↑ Some authors include an additional axiom referred to as the
*closure*under the operation "", which means that is an element of for every and in . This condition is subsumed by requiring "" to be a binary operation on . See Lang 2002. - ↑ The MathSciNet database of mathematics publications lists 1,779 research papers on group theory and its generalizations written in 2020 alone. See MathSciNet 2021.
- ↑ See, for example, Lang 2002 , Lang 2005, Herstein 1996 and Herstein 1975.
- ↑ The word homomorphism derives from Greek ὁμός—the same and μορφή—structure. See Schwartzman 1994, p. 108.
- ↑ However, a group is not determined by its lattice of subgroups. See Suzuki 1951.
- ↑ The fact that the group operation extends this canonically is an instance of a universal property.
- ↑ Injective and surjective maps correspond to mono- and epimorphisms, respectively. They are interchanged when passing to the dual category.
- ↑ See the Seifert–Van Kampen theorem for an example.
- ↑ An example is group cohomology of a group which equals the singular cohomology of its classifying space, see Weibel 1994, §8.2.
- ↑ Elements which do have multiplicative inverses are called units, see Lang 2002, p. 84, §II.1 .
- ↑ The transition from the integers to the rationals by including fractions is generalized by the field of fractions.
- ↑ The same is true for any field
*F*instead of . See Lang 2005, p. 86, §III.1. - ↑ For example, a finite subgroup of the multiplicative group of a field is necessarily cyclic. See Lang 2002, Theorem IV.1.9 . The notions of torsion of a module and simple algebras are other instances of this principle.
- ↑ The stated property is a possible definition of prime numbers. See
*Prime element*. - ↑ For example, the Diffie–Hellman protocol uses the discrete logarithm. See Gollmann 2011, §15.3.2.
- ↑ The additive notation for elements of a cyclic group would be , where is in .
- ↑ More rigorously, every group is the symmetry group of some graph; see Frucht's theorem, Frucht 1939.
- ↑ More precisely, the monodromy action on the vector space of solutions of the differential equations is considered. See Kuga 1993, pp. 105–113.
- ↑ This was crucial to the classification of finite simple groups, for example. See Aschbacher 2004.
- ↑ See, for example, Schur's Lemma for the impact of a group action on simple modules. A more involved example is the action of an absolute Galois group on étale cohomology.
- ↑ The groups of order at most 2000 have been tabulated: up to isomorphism, there are about 49 billion. See Besche, Eick & O'Brien 2001.
- ↑ The gap between the classification of simple groups and the one of all groups lies in the extension problem, a problem too hard to be solved in general. See Aschbacher 2004, p. 737.
- ↑ Equivalently, a nontrivial group is simple if its only quotient groups are the trivial group and the group itself. See Michler 2006, Carter 1989.
- ↑ See Schwarzschild metric for an example where symmetry greatly reduces the complexity of physical systems.

- ↑ Herstein 1975, p. 26, §2.
- ↑ Hall 1967, p. 1, §1.1: "The idea of a group is one which pervades the whole of mathematics both pure and applied."
- ↑ Lang 2005, p. 360, App. 2.
- ↑ Cook 2009, p. 24.
- ↑ Artin 2018, p. 40, §2.2.
- ↑ Lang 2002, p. 3, I.§1 and p. 7, I.§2.
- ↑ Lang 2005, p. 16, II.§1.
- ↑ Herstein 1975, p. 54, §2.6.
- ↑ Wussing 2007.
- ↑ Kleiner 1986.
- ↑ Smith 1906.
- ↑ Galois 1908.
- ↑ Kleiner 1986, p. 202.
- ↑ Cayley 1889.
- ↑ Wussing 2007, §III.2.
- ↑ Lie 1973.
- ↑ Kleiner 1986, p. 204.
- ↑ Wussing 2007, §I.3.4.
- ↑ Jordan 1870.
- ↑ von Dyck 1882.
- ↑ Curtis 2003.
- ↑ Mackey 1976.
- ↑ Borel 2001.
- ↑ Solomon 2018.
- ↑ Ledermann 1953, pp. 4–5, §1.2.
- ↑ Ledermann 1973, p. 3, §I.1.
- ↑ Lang 2002, p. 7, §I.2.
- 1 2 Lang 2005, p. 17, §II.1.
- ↑ Artin 2018, p. 40.
- ↑ Lang 2005, p. 34, §II.3.
- 1 2 Mac Lane 1998.
- ↑ Lang 2005, p. 19, §II.1.
- ↑ Ledermann 1973, p. 39, §II.12.
- ↑ Lang 2005, p. 41, §II.4.
- ↑ Lang 2002, p. 12, §I.2.
- ↑ Lang 2005, p. 45, §II.4.
- ↑ Lang 2002, p. 9, §I.2.
- ↑ Magnus, Karrass & Solitar 2004, pp. 56–67, §1.6.
- ↑ Hatcher 2002, p. 30, Chapter I.
- ↑ Coornaert, Delzant & Papadopoulos 1990.
- ↑ For example, class groups and Picard groups; see Neukirch 1999, in particular §§I.12 and I.13
- ↑ Seress 1997.
- ↑ Lang 2005, Chapter VII.
- ↑ Rosen 2000, p. 54, (Theorem 2.1).
- ↑ Lang 2005, p. 292, §VIII.1.
- ↑ Lang 2005, p. 22, §II.1.
- ↑ Lang 2005, p. 26, §II.2.
- ↑ Lang 2005, p. 22, §II.1 (example 11).
- ↑ Lang 2002, pp. 26, 29, §I.5.
- ↑ Weyl 1952.
- 1 2 Ellis 2019.
- ↑ Conway et al. 2001. See also Bishop 1993
- ↑ Weyl 1950, pp. 197–202.
- ↑ Bersuker 2006, p. 2.
- ↑ Jahn & Teller 1937.
- ↑ Dove 2003.
- ↑ Zee 2010, p. 228.
- ↑ Chancey & O'Brien 2021, pp. 15, 16.
- ↑ Simons 2003, §4.2.1.
- ↑ Eliel, Wilen & Mander 1994, p. 82.
- ↑ Behler, Wickleder & Christoffers 2014, p. 67.
- ↑ Welsh 1989.
- ↑ Mumford, Fogarty & Kirwan 1994.
- ↑ Lay 2003.
- ↑ Kuipers 1999.
- 1 2 Fulton & Harris 1991.
- ↑ Serre 1977.
- ↑ Rudin 1990.
- ↑ Robinson 1996, p. viii.
- ↑ Artin 1998.
- ↑ Lang 2002, Chapter VI (see in particular p. 273 for concrete examples).
- ↑ Lang 2002, p. 292, (Theorem VI.7.2).
- ↑ Stewart 2015, §12.1.
- ↑ Kurzweil & Stellmacher 2004, p. 3.
- ↑ Artin 2018 , Proposition 6.4.3. See also Lang 2002 , p. 77 for similar results.
- ↑ Lang 2002, p. 22, I.§3.
- ↑ Ronan 2007.
- ↑ Awodey 2010, §4.1.
- ↑ Husain 1966.
- ↑ Neukirch 1999.
- ↑ Shatz 1972.
- ↑ Milne 1980.
- ↑ Warner 1983.
- ↑ Borel 1991.
- ↑ Goldstein 1980.
- ↑ Weinberg 1972.
- ↑ Naber 2003.
- ↑ Zee 2010.
- ↑ Denecke & Wismath 2002.
- ↑ Romanowska & Smith 2002.
- ↑ Dudek 2001.

In mathematics, an **abelian group**, also called a **commutative group**, is a group in which the result of applying the group operation to two group elements does not depend on the order in which they are written. That is, the group operation is commutative. With addition as an operation, the integers and the real numbers form abelian groups, and the concept of an abelian group may be viewed as a generalization of these examples. Abelian groups are named after early 19th century mathematician Niels Henrik Abel.

In mathematics, one can often define a **direct product** of objects already known, giving a new one. This generalizes the Cartesian product of the underlying sets, together with a suitably defined structure on the product set. More abstractly, one talks about the product in category theory, which formalizes these notions.

A **quotient group** or **factor group** is a mathematical group obtained by aggregating similar elements of a larger group using an equivalence relation that preserves some of the group structure. For example, the cyclic group of addition modulo *n* can be obtained from the group of integers under addition by identifying elements that differ by a multiple of *n* and defining a group structure that operates on each such class as a single entity. It is part of the mathematical field known as group theory.

In mathematics, a **finite field** or **Galois field** is a field that contains a finite number of elements. As with any field, a finite field is a set on which the operations of multiplication, addition, subtraction and division are defined and satisfy certain basic rules. The most common examples of finite fields are given by the integers mod *p* when *p* is a prime number.

In abstract algebra, a **group isomorphism** is a function between two groups that sets up a one-to-one correspondence between the elements of the groups in a way that respects the given group operations. If there exists an isomorphism between two groups, then the groups are called **isomorphic**. From the standpoint of group theory, isomorphic groups have the same properties and need not be distinguished.

In mathematics, a **group action** on a space is a group homomorphism of a given group into the group of transformations of the space. Similarly, a group action on a mathematical structure is a group homomorphism of a group into the automorphism group of the structure. It is said that the group *acts* on the space or structure. If a group acts on a structure, it will usually also act on objects built from that structure. For example, the group of Euclidean isometries acts on Euclidean space and also on the figures drawn in it. In particular, it acts on the set of all triangles. Similarly, the group of symmetries of a polyhedron acts on the vertices, the edges, and the faces of the polyhedron.

In mathematics, specifically abstract algebra, an **integral domain** is a nonzero commutative ring in which the product of any two nonzero elements is nonzero. Integral domains are generalizations of the ring of integers and provide a natural setting for studying divisibility. In an integral domain, every nonzero element *a* has the cancellation property, that is, if *a* ≠ 0, an equality *ab* = *ac* implies *b* = *c*.

In mathematics, a **Lie group** is a group that is also a differentiable manifold. A manifold is a space that locally resembles Euclidean space, whereas groups define the abstract, generic concept of multiplication and the taking of inverses (division). Combining these two ideas, one obtains a continuous group where points can be multiplied together, and their inverse can be taken. If, in addition, the multiplication and taking of inverses are defined to be smooth (differentiable), one obtains a Lie group.

In mathematics, **modular arithmetic** is a system of arithmetic for integers, where numbers "wrap around" when reaching a certain value, called the **modulus**. The modern approach to modular arithmetic was developed by Carl Friedrich Gauss in his book *Disquisitiones Arithmeticae*, published in 1801.

In mathematics, a **semigroup** is an algebraic structure consisting of a set together with an associative binary operation.

In abstract algebra, the **symmetric group** defined over any set is the group whose elements are all the bijections from the set to itself, and whose group operation is the composition of functions. In particular, the finite symmetric group defined over a finite set of symbols consists of the permutations that can be performed on the symbols. Since there are such permutation operations, the order of the symmetric group is .

In mathematics, **rings** are algebraic structures that generalize fields: multiplication need not be commutative and multiplicative inverses need not exist. In other words, a *ring* is a set equipped with two binary operations satisfying properties analogous to those of addition and multiplication of integers. Ring elements may be numbers such as integers or complex numbers, but they may also be non-numerical objects such as polynomials, square matrices, functions, and power series.

In mathematics, a **free abelian group** or **free Z-module** is an abelian group with a basis, or, equivalently, a free module over the integers. Being an abelian group means that it is a set with an addition operation that is associative, commutative, and invertible. A basis, also called an integral basis, is a subset such that every element of the group can be uniquely expressed as a linear combination of basis elements with integer coefficients. For instance, the integers with addition form a free abelian group with basis {1}. Free abelian groups have properties which make them similar to vector spaces. They have applications in algebraic topology, where they are used to define chain groups, and in algebraic geometry, where they are used to define divisors. Integer lattices also form examples of free abelian groups, and lattice theory studies free abelian subgroups of real vector spaces.

In mathematics, **group cohomology** is a set of mathematical tools used to study groups using cohomology theory, a technique from algebraic topology. Analogous to group representations, group cohomology looks at the group actions of a group *G* in an associated *G*-module*M* to elucidate the properties of the group. By treating the *G*-module as a kind of topological space with elements of representing *n*-simplices, topological properties of the space may be computed, such as the set of cohomology groups . The cohomology groups in turn provide insight into the structure of the group *G* and *G*-module *M* themselves. Group cohomology plays a role in the investigation of fixed points of a group action in a module or space and the quotient module or space with respect to a group action. Group cohomology is used in the fields of abstract algebra, homological algebra, algebraic topology and algebraic number theory, as well as in applications to group theory proper. As in algebraic topology, there is a dual theory called *group homology*. The techniques of group cohomology can also be extended to the case that instead of a *G*-module, *G* acts on a nonabelian *G*-group; in effect, a generalization of a module to non-Abelian coefficients.

In algebra, a **valuation** is a function on a field that provides a measure of size or multiplicity of elements of the field. It generalizes to commutative algebra the notion of size inherent in consideration of the degree of a pole or multiplicity of a zero in complex analysis, the degree of divisibility of a number by a prime number in number theory, and the geometrical concept of contact between two algebraic or analytic varieties in algebraic geometry. A field with a valuation on it is called a **valued field**.

In algebra, a **group ring** is a free module and at the same time a ring, constructed in a natural way from any given ring and any given group. As a free module, its ring of scalars is the given ring, and its basis is one-to-one with the given group. As a ring, its addition law is that of the free module and its multiplication extends "by linearity" the given group law on the basis. Less formally, a group ring is a generalization of a given group, by attaching to each element of the group a "weighting factor" from a given ring.

In mathematics, a **congruence subgroup** of a matrix group with integer entries is a subgroup defined by congruence conditions on the entries. A very simple example would be invertible 2 × 2 integer matrices of determinant 1, in which the off-diagonal entries are *even*. More generally, the notion of **congruence subgroup** can be defined for arithmetic subgroups of algebraic groups; that is, those for which we have a notion of 'integral structure' and can define reduction maps modulo an integer.

In mathematics, the **Grothendieck group** construction constructs an abelian group from a commutative monoid *M* in the most universal way, in the sense that any abelian group containing a homomorphic image of *M* will also contain a homomorphic image of the Grothendieck group of *M*. The Grothendieck group construction takes its name from a specific case in category theory, introduced by Alexander Grothendieck in his proof of the Grothendieck–Riemann–Roch theorem, which resulted in the development of K-theory. This specific case is the monoid of isomorphism classes of objects of an abelian category, with the direct sum as its operation.

In mathematics, specifically in category theory, *F*-**algebras** generalize the notion of algebraic structure. Rewriting the algebraic laws in terms of morphisms eliminates all references to quantified elements from the axioms, and these algebraic laws may then be glued together in terms of a single functor *F*, the *signature*.

In mathematics, a **near-field** is an algebraic structure similar to a division ring, except that it has only one of the two distributive laws. Alternatively, a near-field is a near-ring in which there is a multiplicative identity and every non-zero element has a multiplicative inverse.

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