Definitions

# group

[groop]
group, in mathematics, system consisting of a set of elements and a binary operation ab defined for combining two elements such that the following requirements are satisfied: (1) The set is closed under the operation; i.e., if a and b are elements of the set, then the element that results from combining a and b under the operation is also an element of the set; (2) the operation satisfies the associative law; i.e., a∘(bc)=(ab)∘c, where ∘ represents the operation and a, b, and c are any three elements; (3) there exists an identity element I in the set such that aI=a for any element a in the set; (4) there exists an inverse a-1 in the set for every a such that aa-1=I. If, in addition to satisfying these four axioms, the group also satisfies the commutative law for the operation, i.e., ab=ba, then it is called a commutative, or Abelian, group. The real numbers (see number) form a commutative group both under addition, with 0 as identity element and -a as inverse, and, excluding 0, under multiplication, with 1 as identity element and 1/a as inverse. The elements of a group need not be numbers; they may often be transformations, or mappings, of one set of objects into another. For example, the set of all permutations of a finite collection of objects constitutes a group. Group theory has wide applications in mathematics, including number theory, geometry, and statistics, and is also important in other branches of science, e.g., elementary particle theory and crystallography.

See R. P. Burn, Groups (1987); J. A. Green, Sets and Groups (1988).

or pressure group

any association of individuals or organizations, usually formally organized, that, on the basis of one or more shared concerns, attempts to influence public policy in its favour. All interest groups share a desire to affect government policy to benefit themselves or their cause. It could be a policy that exclusively benefits group members or one segment of society (e.g., government subsidies for farmers) or a policy that advances a broader public purpose (e.g., improving air quality). Interest groups are a natural outgrowth of the communities of interests that exist in all societies, from the narrowest groups such as the Japan Eraser Manufacturers Association to broader groups such as the AFL-CIO to very broad organizations such as the military in authoritarian countries. Interest groups exist at all levels of government—national, state, provincial, and local—and increasingly they have occupied an important role in international affairs.

All the genes on a single chromosome. They are inherited as a group; during cell division they act and move as a unit rather than independently. Variations in linkage groups can occur if a chromosome breaks, and the sections join with the partner chromosome if it has broken in the same places. This exchange of genes between chromosomes, called crossing-over, usually occurs during meiosis. Sex linkage is the tendency of a characteristic to be linked to one sex; sex-linked traits in humans include red-green colour blindness and hemophilia.

Form of psychotherapy in which several patients or clients discuss their personal problems, usually in the presence of a therapist or counselor. In one approach to group therapy, the chief aim is to raise members' awareness and morale and combat feelings of isolation by cultivating a sense of belonging to the group; a notable example is Alcoholics Anonymous. The other principal approach strives to foster free discussion and uninhibited self-revelation; members are helped to self-understanding and more successful behaviour through mutual examination of their reactions to people in their lives, including one another.

In molecules, any of numerous combinations of atoms that undergo characteristic chemical reactions themselves and in many cases influence the reactivity of the rest of the molecule. Organic compounds are often classified according to the functional groups they contain. Common functional groups include hydroxyl (singlehorzbondOH), in alcohols and phenols; carboxyl (singlehorzbondCOOH), in carboxylic acids; carbonyl (singlehorzbondCdoublehorzbondO), in aldehydes, ketones, amides, carboxylic acids, esters, and quinones; and nitro (singlehorzbondNO2) and amino (singlehorzbondNH2), in certain organic nitrogen compounds.

Social group or category of the population that, in a larger society, is set apart and bound together by common ties of language, nationality, or culture. Ethnic diversity, the legacy of political conquests and migrations, is one aspect of the social complexity found in most contemporary societies. The nation-state has traditionally been uneasy with ethnic diversity, and nation-states have often attempted to eliminate or expel ethnic groups. Most nations today practice some form of pluralism, which usually rests on a combination of toleration, interdependence, and separatism. The concept of ethnicity is more important today than ever, as a result of the spread of doctrines of freedom, self-determination, and democracy. Seealso culture contact; ethnic cleansing; ethnocentrism; race; racism.

Toronto-centred group of Canadian painters devoted to landscape painting (especially of northern Ontario subjects) and the creation of a national style. A number of future members met in 1913 while working as commercial artists in Toronto. The group adopted its name on the occasion of a group exhibition held in 1920. The original members included J.E.H. MacDonald, Lawren S. Harris, Arthur Lismer, F.H. Varley, Franklin Carmichael, Frank H. Johnston, and Alexander Young Jackson. The group was particularly influential in the 1920s and '30s. In 1933 the name was changed to the Canadian Group of Painters.

Concentration of about 50 galaxies to which the Milky Way Galaxy belongs. Nearly one-third are dwarf elliptical galaxies, but the six largest are spiral or irregular galaxies. They are probably kept from separating by mutual gravitational attraction. The Milky Way system is near one end of the group; the great Andromeda Galaxy is near the other end, about two million light-years away.

New York theatre company (1931–41) founded by Harold Clurman, Cheryl Crawford, and Lee Strasberg to present U.S. plays of social significance. Embracing the acting principles of the Stanislavsky method, the company—which also included actors and directors such as Elia Kazan, Lee J. Cobb, and Stella Adler—staged John Howard Lawson's Success Story (1932), Sidney Kingsley's Men in White (1933), Clifford Odets's Waiting for Lefty (1935) and Golden Boy (1937), Irwin Shaw's Bury the Dead (1936), and William Saroyan's My Heart's in the Highlands (1939), among many other plays.

A coterie of English writers, philosophers, and artists. The name was a reference to the Bloomsbury district of London, where between about 1907 and 1930 the group frequently met to discuss aesthetic and philosophical questions. Among the group were E.M. Forster, Lytton Strachey, Clive Bell, the painters Vanessa Bell (1879–1961) and Duncan Grant (1885–1978), John Maynard Keynes, the Fabian writer Leonard Woolf (1880–1969), and Virginia Woolf.

In mathematics, given a prime number p, a p-group is a periodic group in which each element has a power of p as its order. That is, for each element g of the group, there exists a nonnegative integer n such that g to the power pn is equal to the identity element. Such groups are also called primary.

The remainder of this article deals with finite p-groups. For an example of an infinite abelian p-group, see Prüfer group, and for an example of an infinite simple p-group, see Tarski monster group.

A finite group is a p-group if and only if its order (the number of its elements) is a power of p.

## Properties

Quite a lot is known about the structure of finite p-groups.

### Non-trivial center

One of the first standard results using the class equation is that the center of a non-trivial finite p-group cannot be the trivial subgroup (proof).

This forms the basis for many inductive methods in p-groups.

For instance, the normalizer N of a proper subgroup H of a finite p-group G properly contains H, because for any counterexample with H=N, the center Z is contained in N, and so also in H, but then there is a smaller example H/Z whose normalizer in G/Z is N/Z=H/Z, creating an infinite descent. As a corollary, every finite p-group is nilpotent.

In another direction, every normal subgroup of a finite p-group intersects the center nontrivially. In particular, every minimal normal subgroup of a finite p-group is of order p and contained in the center. Indeed, the socle of a finite p-group is the subgroup of the center consisting of the central elements of order p.

If G is a p-group, then so is G/Z, and so it too has a nontrivial center. The preimage in G of the center of G/Z is called the second center and these groups begin the upper central series. Generalizing the earlier comments about the socle, a finite p-group with order pn contains normal subgroups of order pi with 0 ≤ in, and any normal subgroup of order pi is contained in the ith center Zi. If a normal subgroup is not contained in Zi, then its intersection with Zi+1 has size at least pi+1.

### Automorphisms

The automorphism groups of p-groups are well studied. Just as every finite p-group has a nontrivial center so that the inner automorphism group is a proper quotient of the group, every finite p-group has a nontrivial outer automorphism group. Every automorphism of G induces an automorphism on G/Φ(G), where Φ(G) is the Frattini subgroup of G. The quotient G/Φ(G) is an elementary abelian group and its automorphism group is a general linear group, so very well understood. The map from the automorphism group of G into this general linear group has been studied by Burnside, who showed that the kernel of this map is a p-group.

## Examples

p-groups of the same order are not necessarily isomorphic; for example, the cyclic group C4 and the Klein group V4 are both 2-groups of order 4, but they are not isomorphic.

Nor need a p-group be abelian; the dihedral group Dih4 of order 8 is a non-abelian 2-group. However, every group of order p2 is abelian.

The dihedral groups are both very similar to and very dissimilar from the quaternion groups and the semidihedral groups. Together the dihedral, semidihedral, and quaternion groups form the 2-groups of maximal class, that is those groups of order 2n+1 and nilpotency class n.

### Iterated wreath products

The iterated wreath products of cyclic groups of order p are very important examples of p-groups. Denote the cyclic group of order p as W(1), and the wreath product of W(n) with W(1) as W(n+1). Then W(n) is the Sylow p-subgroup of the symmetric group Sym(pn). Maximal p-subgroups of the general linear group GL(n,Q) are direct products of various W(n). It has order pk where k=(pn−1)/(p−1). It has nilpotency class pn−1, and its lower central series, upper central series, lower exponent-p central series, and upper exponent-p central series are equal. It is generated by its elements of order p, but its exponent is pn. The second such group, W(2), is also a p-group of maximal class, since it has order pp+1 and nilpotency class p, but is not a regular p-group. Since groups of order pp are always regular groups, it is also a minimal such example.

### Generalized dihedral groups

When p=2 and n=2, W(n) is the dihedral group of order 8, so in some sense W(n) provides an analogue for the dihedral group for all primes p when n=2. However, for higher n the analogy becomes strained. There is a different family of examples that more closely mimics the dihedral groups of order 2n, but that requires a bit more setup. Let ζ denote a primitive pth root of unity in the complex numbers, and let Z[ζ] be the ring of cyclotomic integers generated by it, and let P be the prime ideal generated by 1−ζ. Let G be a cyclic group of order p generated by an element z. Form the semidirect product E(p) of Z[ζ] and G where z acts as multiplication by ζ. The powers Pn are normal subgroups of E(p), and the example groups are E(p,n) = E(p)/Pn. E(p,n) has order pn+1 and nilpotency class n, so is a p-group of maximal class. When p=2, E(2,n) is the dihedral group of order 2n. When p is odd, both W(2) and E(p,p) are irregular groups of maximal class and order pp+1, but are not isomorphic.

### Unitriangular matrix groups

The Sylow subgroups of general linear groups are another fundamental family of examples. Let V be a vector space of dimension n with basis { e1, e2, …, en } and define Vi to be the vector space generated by { ei, ei+1, …, en } for 1 ≤ in, and define Vi = 0 when i > n. For each 1 ≤ mn, the set of invertible linear transformations of V which take each Vi to Vi+m form a subgroup of Aut(V) denoted Um. If V is a vector space over Z/pZ, then U1 is a Sylow p-subgroup of Aut(V) = GL(n, p), and the terms of its lower central series are just the Um. In terms of matrices, Um are those upper triangular matrices with 1s one the diagonal and 0s on the first m−1 superdiagonals. The group U1 has order pn·(n−1)/2, nilpotency class n, and exponent pk where k is the least integer at least as large as the base p logarithm of n.

## Classification

The groups of order pn for 0 ≤ n ≤ 4 were classified early in the history of group theory , and modern work has extended these classifications to groups whose order divides p7, though the sheer number of families of such groups grows so quickly that further classifications along these lines are judged difficult for the human mind to comprehend .

Rather than classify the groups by order, Philip Hall proposed using a notion of isoclinism which gathered finite p-groups into families based on large quotient and subgroups .

An entirely different method classifies finite p-groups by their coclass, that is, the difference between their composition length and their nilpotency class. The so-called coclass conjectures described the set of all finite p-groups of fixed coclass as perturbations of finitely many pro-p groups. The coclass conjectures were proven in the 1980s using techniques related to Lie algebras and powerful p-groups .

## Prevalence

In an asymptotic sense, almost all finite groups are p-groups. In fact, almost all finite groups are 2-groups: the fraction of isomorphism classes of 2-groups among isomorphism classes of groups of order at most n tends to 1 as n tends to infinity. For instance, of the 49 910 529 484 different groups of order at most 2000, 49 487 365 422, or just over 99%, are 2-groups of order 1024 .

Every finite group whose order is divisible by p contains a subgroup which is a non-trivial p-group, namely a cyclic group of order p generated by an element of order p obtained from Cauchy's theorem, or a larger p-subgroup obtained from Sylow's theorem.

## Local control

Much of the structure of a finite group is carried in the structure of its so-called local subgroups, the normalizers of non-identity p-subgroups .

The large elementary abelian subgroups of a finite group exert control over the group that was used in the proof of the Feit-Thompson theorem. Certain central extensions of elementary abelian groups called extraspecial groups help describe the structure of groups as acting symplectic vector spaces.

Brauer classified all groups whose Sylow 2-subgroups are the direct product of two cyclic groups of order 4, and Walter, Gorenstein, Bender, Suzuki, Glauberman, and others classified those simple groups whose Sylow 2-subgroups were abelian, dihedral, semidihedral, or quaternion.