Group theory is that branch of mathematics concerned with the study of groups.
Please refer to the Glossary of group theory for the definitions of terms used throughout group theory.
See also list of group theory topics.
History
There are three historical roots of group theory: the theory of algebraic equations, number theory and geometry. Euler, Gauss, Lagrange, Abel and Galois were early researchers in the field of group theory. Galois is honored as the first mathematician linking group theory and field theory, with the theory that is now called Galois theory.
An early source occurs in the problem of forming an mthdegree equation having as its roots m of the roots of a given nthdegree equation (m < n). For simple cases the problem goes back to Hudde (1659). Saunderson (1740) noted that the determination of the quadratic factors of a biquadratic expression necessarily leads to a sextic equation, and Le Sœur (1748) and Waring (1762 to 1782) still further elaborated the idea.
A common foundation for the theory of equations on the basis of the group of permutations was found by Lagrange (1770, 1771), and on this was built the theory of substitutions. He discovered that the roots of all resolvents (résolvantes, réduites) which he examined are rational functions of the roots of the respective equations. To study the properties of these functions he invented a Calcul des Combinaisons. The contemporary work of Vandermonde (1770) also foreshadowed the coming theory.
Ruffini (1799) attempted a proof of the impossibility of solving the quintic and higher equations. Ruffini distinguished what are now called intransitive and transitive, and imprimitive and primitive groups, and (1801) uses the group of an equation under the name l'assieme della permutazioni. He also published a letter from Abbati to himself, in which the group idea is prominent.
Galois found that if are the n roots of an equation, there is always a group of permutations of the r's such that (1) every function of the roots invariable by the substitutions of the group is rationally known, and (2), conversely, every rationally determinable function of the roots is invariant under the substitutions of the group. Galois also contributed to the theory of modular equations and to that of elliptic functions. His first publication on the group theory was made at the age of eighteen (1829), but his contributions attracted little attention until the publication of his collected papers in 1846 (Liouville, Vol. XI).
Arthur Cayley and Augustin Louis Cauchy were among the first to appreciate the importance of the theory, and to the latter especially are due a number of important theorems. The subject was popularised by Serret , who devoted section IV of his algebra to the theory; by Camille Jordan, whose Traité des Substitutions is a classic; and to Netto (1882), whose was translated into English by Cole (1892). Other group theorists of the nineteenth century were Bertrand, Charles Hermite, Frobenius, Leopold Kronecker, and Emile Mathieu.
It was Walther von Dyck who, in 1882, gave the modern definition of a group.
The study of what are now called Lie groups, and their discrete subgroups, as transformation groups, started systematically in 1884 with Sophus Lie; followed by work of Killing, Study, Schur and Maurer . The discontinuous (discrete group) theory was built up by Felix Klein, Lie, Poincaré, and Charles Emile Picard, in connection in particular with modular forms and monodromy.
Other important mathematicians in this subject area include Emil Artin, Emmy Noether, Sylow, and many others.
Elementary introduction
Groups are used throughout mathematics and the sciences, often to capture the internal symmetry of other structures, in the form of automorphism groups. An internal symmetry of a structure is usually associated with an invariant property, and the set of transformations that preserve this invariant property, together with the operation of composition of transformations, form a group called a symmetry group
In Galois theory, which is the historical origin of the group concept, one uses groups to describe the symmetries of the equations satisfied by the solutions to a polynomial equation. The solvable groups are sonamed because of their prominent role in this theory.
Abelian groups underlie several other structures that are studied in abstract algebra, such as rings, fields, and modules.
In algebraic topology, groups are used to describe invariants of topological spaces (the name of the torsion subgroup of an infinite group shows the legacy of this field of endeavor). They are called "invariants" because they are defined in such a way that they don't change if the space is subjected to some deformation. Examples include the fundamental group, homology groups and cohomology groups.
The concept of the Lie group (named for mathematician Sophus Lie) is important in the study of differential equations and manifolds; they combine analysis and group theory and are therefore the proper objects for describing symmetries of analytical structures. Analysis on these and other groups is called harmonic analysis.
In combinatorics, the notion of permutation group and the concept of group action are often used to simplify the counting of a set of objects; see in particular Burnside's lemma.
An understanding of group theory is also important in the physical sciences. In chemistry, groups are used to classify crystal structures, regular polyhedra, and the symmetries of molecules. In physics, groups are important because they describe the symmetries which the law of physics seem to obey. Physicists are very interested in group representations, especially of Lie groups, since these representations often point the way to the "possible" physical theories.

Physics examples: Standard Model, Gauge_theory
Some useful theorems
Generalizations
In abstract algebra, we get some related structures which are similar to groups by relaxing some of the axioms given at the top of the article.
 If we eliminate the requirement that every element have an inverse, then we get a monoid.
 If we additionally do not require an identity either, then we get a semigroup.
 Alternatively, if we relax the requirement that the operation be associative while still requiring the possibility of division, then we get a loop.
 If we additionally do not require an identity, then we get a quasigroup.
 If we don't require any axioms of the binary operation at all, then we get a magma.
Groupoids, which are similar to groups except that the composition a * b need not be defined for all a and b, arise in the study of more involved kinds of symmetries, often in topological and analytical structures. They are special sorts of categories.
Supergroups and Hopf algebras are other generalizations.
Lie groups, algebraic groups and topological groups are examples of group objects: grouplike structures sitting in a category other than the ordinary category of sets.
Abelian groups form the prototype for the concept of an abelian category, which has applications to vector spaces and beyond.
Formal group laws are certain formal power series which have properties much like a group operation.
Miscellany
James Newman summarized group theory as follows:
 The theory of groups is a branch of mathematics in which one does something to something and then compares the results with the result of doing the same thing to something else, or something else to the same thing.
One application of group theory is in musical set theory.
External link