Finite group

In mathematics, a finite group is a group which has finitely many elements. Some aspects of the theory of finite groups were investigated in great depth in the twentieth century, in particular the local theory, and the theory of solvable groups and nilpotent groups. It is too much to hope for a complete determination of the structure of all finite groups: the number of possible structures soon becomes overwhelming. However, the twentieth century saw the classification of the finite simple groups, which may be viewedas the determination of the "building blocks" for all finite groups,as each finite group has a composition series.

Thanks to the work of mathematicians such Chevalley and Steinberg, the second half of the twentieth century also saw increased understanding of finite analogs of classical groups, and other related groups. One such familyof groups is the family of general linear groups over finite fields. The group theorist [http://www.math.uchicago.edu/~alperin/ J. L. Alperin] has written that "The typical example of a finite group is GL(n,q), the general linear group of n dimensions over the field with q elements. The student who is introduced to the subject with other examples is being completely misled." [Jonathan L. Alperin, Book review: B. Huppert and N. Blackburn Title: Finite groups, Bulletin (New Series) of the American Mathematical Society, 10 (1984) 121, DOI|10.1090/S0273-0979-1984-15210-8]

Finite groups often occur when considering symmetry of mathematical orphysical objects, when those objects admit just a finite number of structure preserving transformations. The theory of Lie groups,which may be viewed as dealing with "continuous symmetry", is strongly influenced by the associated Weyl groups. These are finite groups generated by reflections which act on a finite dimensional
Euclidean space. Thus properties of finite groups can play a role in subjects such as theoretical physics.

Number of groups of a given order

Given a positive integer "n", it is not at all a routine matter to determine how many isomorphism types of groups of order "n" there are. Every group of prime order is cyclic, since Lagrange's theorem implies that the cyclic subgroup generated by any of its non-identity elements is the whole group.If "n" is the square of a prime, then there are exactly two possible isomorphism types of group of order "n", both of which are abelian. If "n" is a higher power of a prime, then results of
Graham Higman and Charles Sims give asymptotically correct estimates for the numberof isomorphism types of groups of order "n", and the number grows very rapidly as the power increases.

Depending on the prime factorization of "n," some restrictions may be placed on the structure of groups of order "n," as a consequence, for example, of results such as the Sylow theorems. For example, every group of order "pq" is cyclic when "q" < "p" are primes with "p-1" not divisible by "q". If "n" issquarefree, then any group of order "n" is solvable. A theorem of William Burnside, proved using group characters, states that every group of order "n" is solvable when "n" is divisible by fewer than three distinct primes. By the Feit-Thompson theorem, which has a long and complicated proof, every group of order "n" is solvable when "n" is odd.

There is a meaningful sense in which for every positive integer "n", most groups of order "n" are solvable. To see this for any particular order is usually not difficult (for example, there is (up to isomorphism) only one non-solvable group of order "60", while there are two non-isomorphic abelian groups of order "60" and several more isomorphism types of non-abelian solvable groups of order "60") but to make such a statement precise for all "n" requires the classification of finite simple groups. Without the classification theorem, it is not clear whether there is a constant bounding the number of isomorphism types of simple groups of order "n" (with the benefit of the classification, it is known that the constant "2" is an upper bound for all "n". Prior to the classification, it had long been known that there were infinitely many values of "n" for which two non-isomorphic simple groups of order "n" existed).

References

ee also

*Classification of finite simple groups
*List of finite simple groups
*Lagrange's theorem
*Cauchy's theorem (group theory)
*P-group
*List of small groups
*Representation theory of finite groups
*Modular representation theory
*Monstrous moonshine
*Profinite group
*Infinite group theory