# Portal:Algebra

## Introduction

**Algebra** (from Arabic *"al-jabr"*, literally meaning "reunion of broken parts") is one of the broad parts of mathematics, together with number theory, geometry and analysis. In its most general form, algebra is the study of mathematical symbols and the rules for manipulating these symbols; it is a unifying thread of almost all of mathematics. As such, it includes everything from elementary equation solving to the study of abstractions such as groups, rings, and fields. The more basic parts of algebra are called elementary algebra; the more abstract parts are called abstract algebra or modern algebra. Elementary algebra is generally considered to be essential for any study of mathematics, science, or engineering, as well as such applications as medicine and economics. Abstract algebra is a major area in advanced mathematics, studied primarily by professional mathematicians.

Elementary algebra differs from arithmetic in the use of abstractions, such as using letters to stand for numbers that are either unknown or allowed to take on many values. For example, in the letter is unknown, but the law of inverses can be used to discover its value: . In *E* = *mc*^{2}, the letters and are variables, and the letter is a constant, the speed of light in a vacuum. Algebra gives methods for writing formulas and solving equations that are much clearer and easier than the older method of writing everything out in words.

The word *algebra* is also used in certain specialized ways. A special kind of mathematical object in abstract algebra is called an "algebra", and the word is used, for example, in the phrases linear algebra and algebraic topology.

## Selected article

In this shear transformation of the Mona Lisa, the central vertical axis (red vector) is unchanged, but the diagonal vector (blue) has changed direction. Hence the red vector is said to be an eigenvector of this particular transformation and the blue vector is not. |

In mathematics, an **eigenvector** of a transformation is a vector which that transformation simply multiplies by a constant factor, called the **eigenvalue** of that vector. Often, a transformation is completely described by its eigenvalues and eigenvectors. The **eigenspace** for a factor is the set of eigenvectors with that factor as eigenvalue.

In the specific case of linear algebra, the *eigenvalue problem* is this: given an *n* by *n* matrix *A*,what nonzero vectors *x* in exist, such that *Ax* is a scalar multiple of *x*?

The scalar multiple is denoted by the Greek letter *λ* and is called an *eigenvalue* of the matrix A, while *x* is called the *eigenvector* of *A* corresponding to *λ*. These concepts play a major role in several branches of both pure and applied mathematics — appearing prominently in linear algebra, functional analysis, and to a lesser extent in nonlinear situations.

It is common to prefix any natural name for the vector with *eigen* instead of saying *eigenvector*. For example, *eigenfunction* if the eigenvector is a function, *eigenmode* if the eigenvector is a harmonic mode, *eigenstate* if the eigenvector is a quantum state, and so on. Similarly for the eigenvalue, e.g. *eigenfrequency* if the eigenvalue is (or determines) a frequency.

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## Selected picture

These are all the connected **Dynkin diagrams**, which classify the irreducible root systems, which themselves classify simple complex Lie algebras and simple complex Lie groups. These diagrams are therefore fundamental throughout Lie group theory.

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## Did you know?

- ...that it is impossible to devise a single formula involving only polynomials and radicals for solving an arbitrary quintic equation?
- ...that it is possible for a three-dimensional figure to have a finite volume but infinite surface area? An example of this is Gabriel's Horn.
- ...that the Gudermannian function relates the regular trigonometric functions and the hyperbolic trigonometric functions without the use of complex numbers?
- ...that the classification of finite simple groups was not completed until the mid 1980s?
- ...that a field is an algebraic structure in which the operations of addition, subtraction, multiplication and division (except division by zero) may be performed, and the same rules hold which are familiar from the arithmetic of ordinary numbers?
- ...that the classification of finite simple groups is of more than 10000 pages.

*Topics in algebra*

General | Elementary algebra | Key concepts | Linear algebra |
---|---|---|---|

Algebraic structures | Groups | Rings and Fields | Other |

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