# Algebraic number

In mathematics, an algebraic number relative to a field F is any element x of a given field K containing F such that x is a solution of a polynomial equation of the form

anxn + an−1xn−1 + ··· + a1x + a0 = 0

where n is a positive integer called the degree of the polynomial, every coefficient ai is an element of F, and an is nonzero. If the field F is the field Q of rational numbers and K is an algebraically closed field then the algebraic numbers relative to Q are simply called algebraic numbers. The algebraically closed field in which these numbers lie can be the complex numbers C, but sometimes other fields are used. Any such algebraic closure is unique up to field isomorphism, but may differ in topological properties. Considered purely as a field it is unique, and it is either this abstract field devoid of topology or the closure of the rationals in the complex numbers which is most often called the field of algebraic numbers.

All rationals are algebraic. A real number that is not rational may or may not be algebraic; for example irrational numbers such as 21/2 (the square root of 2) and 31/3/2 (the cube root of 3 divided by 2) are also algebraic because they are the solutions of x2 − 2 = 0 and 8x3 − 3 = 0, respectively. But most real numbers are not algebraic; examples of this are π and e. If a complex number is not an algebraic number then it is called a transcendental number. So, for instance i, the imaginary unit, is an algebraic number since it satisfies x2 + 1 = 0; however $\displaystyle i^i$ is transcendental by the Gelfond-Schneider theorem; one branch of this number is e-π/2, which shows that eπ is also transcendental.

If an algebraic number satisfies such an equation as given above with a polynomial of degree n and not such an equation with a lower degree, then the number is said to be an algebraic number of degree n.

## The field of algebraic numbers

The sum, difference, product and quotient of two algebraic numbers is again algebraic, and the algebraic numbers therefore form a field, called the algebraic closure of the field of algebraic numbers. It can be shown that if we allow the coefficients ai to be any algebraic numbers then every solution of the equation will again be an algebraic number. This can be rephrased by saying that the field of algebraic numbers is algebraically closed. In fact, it is the smallest algebraically closed field containing the rationals, and is therefore called the algebraic closure of the rationals.