Algebraic number

An algebraic number is any complex number that is a root of a non-zero polynomial in one variable with rational coefficients (or equivalently – by clearing denominators – with integer coefficients). All integers and rational numbers are algebraic, as are all roots of integers. The same is not true for all real and complex numbers because of transcendental numbers such as π and e. Almost all real and complex numbers are transcendental.^[1]

Examples

• The rational numbers, expressed as the quotient of two integers a and b, b not equal to zero, satisfy the above definition because x = a/b is the root of bx − a.^[2]
• The quadratic surds (irrational roots of a quadratic polynomial ax² + bx + c with integer coefficients a, b, and c) are algebraic numbers. If the quadratic polynomial is monic (a = 1) then the roots are quadratic integers.
• The constructible numbers are those numbers that can be constructed from a given unit length using straightedge and compass. These include all quadratic surds, all rational numbers, and all numbers that can be formed from these using the basic arithmetic operations and the extraction of square roots. (Note that by designating cardinal directions for 1, −1, i, and −i, complex numbers such as 3 + √2i are considered constructible.)
• Any expression formed from algebraic numbers using any combination of the basic arithmetic operations and extraction of nth roots gives another algebraic number.
• Polynomial roots that cannot be expressed in terms of the basic arithmetic operations and extraction of nth roots (such as the roots of x⁵ − x + 1). This happens with many, but not all, polynomials of degree 5 or higher.
• Gaussian integers: those complex numbers a + bi where both a and b are integers are also quadratic integers.
• Trigonometric functions of rational multiples of π (except when undefined): that is, the trigonometric numbers. For example, each of cos π/7, cos 3π/7, cos 5π/7 satisfies 8x³ − 4x² − 4x + 1 = 0. This polynomial is irreducible over the rationals, and so these three cosines are conjugate algebraic numbers. Likewise, tan 3π/16, tan 7π/16, tan 11π/16, tan 15π/16 all satisfy the irreducible polynomial x⁴ − 4x³ − 6x² + 4x + 1, and so are conjugate algebraic integers.
• Some irrational numbers are algebraic and some are not:
  • The numbers √2 and ³√3/2 are algebraic since they are roots of polynomials x² − 2 and 8x³ − 3, respectively.
  • The golden ratio φ is algebraic since it is a root of the polynomial x² − x − 1.
  • The numbers π and e are not algebraic numbers (see the Lindemann–Weierstrass theorem);^[3] hence they are transcendental.

Properties

Algebraic numbers on the complex plane colored by degree (red=1, green=2, blue=3, yellow=4)

• The set of algebraic numbers is countable (enumerable).^[4][5]
• Hence, the set of algebraic numbers has Lebesgue measure zero (as a subset of the complex numbers), i.e. "almost all" complex numbers are not algebraic.
• Given an algebraic number, there is a unique monic polynomial (with rational coefficients) of least degree that has the number as a root. This polynomial is called its minimal polynomial. If its minimal polynomial has degree n, then the algebraic number is said to be of degree n. An algebraic number of degree 1 is a rational number. A real algebraic number of degree 2 is a quadratic irrational.
• All algebraic numbers are computable and therefore definable and arithmetical.
• The set of real algebraic numbers is linearly ordered, countable, densely ordered, and without first or last element, so is order-isomorphic to the set of rational numbers.
• For real numbers a and b, the complex number a + bi is algebraic if and only if both a and b are algebraic.^[6]

The field of algebraic numbers

Algebraic numbers colored by degree (blue=4, cyan=3, red=2, green=1). The unit circle is black.

The sum, difference, product and quotient (if the denominator is nonzero) of two algebraic numbers is again algebraic (this fact can be demonstrated using the resultant), and the algebraic numbers therefore form a field Q (sometimes denoted by A, though this usually denotes the adele ring). Every root of a polynomial equation whose coefficients are algebraic numbers is again algebraic. 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.

The set of real algebraic numbers itself forms a field.^[7]

Related fields

Numbers defined by radicals

All numbers that can be obtained from the integers using a finite number of integer additions, subtractions, multiplications, divisions, and taking nth roots where n is a positive integer (i.e., radical expressions) are algebraic. The converse, however, is not true: there are algebraic numbers that cannot be obtained in this manner. All of these numbers are roots of polynomials of degree ≥5. This is a result of Galois theory (see Quintic equations and the Abel–Ruffini theorem). An example of such a number is the unique real root of the polynomial x⁵ − x − 1 (which is approximately 1.167304).

Closed-form number

Algebraic numbers are all numbers that can be defined explicitly or implicitly in terms of polynomials, starting from the rational numbers. One may generalize this to "closed-form numbers", which may be defined in various ways. Most broadly, all numbers that can be defined explicitly or implicitly in terms of polynomials, exponentials, and logarithms are called "elementary numbers", and these include the algebraic numbers, plus some transcendental numbers. Most narrowly, one may consider numbers explicitly defined in terms of polynomials, exponentials, and logarithms – this does not include all algebraic numbers, but does include some simple transcendental numbers such as e or log(2).

Algebraic integers

Algebraic numbers colored by leading coefficient (red signifies 1 for an algebraic integer)

An algebraic integer is an algebraic number that is a root of a polynomial with integer coefficients with leading coefficient 1 (a monic polynomial). Examples of algebraic integers are 5 + 13√2, 2 − 6i, and 1/2(1 + i√3). Note, therefore, that the algebraic integers constitute a proper superset of the integers, as the latter are the roots of monic polynomials x − k for all k ∈ Z. In this sense, algebraic integers are to algebraic numbers what integers are to rational numbers.

The sum, difference and product of algebraic integers are again algebraic integers, which means that the algebraic integers form a ring. The name algebraic integer comes from the fact that the only rational numbers that are algebraic integers are the integers, and because the algebraic integers in any number field are in many ways analogous to the integers. If K is a number field, its ring of integers is the subring of algebraic integers in K, and is frequently denoted as O_K. These are the prototypical examples of Dedekind domains.

Special classes of algebraic number

• Algebraic solution
• Gaussian integer
• Eisenstein integer
• Quadratic irrational
• Fundamental unit
• Root of unity
• Gaussian period
• Pisot–Vijayaraghavan number
• Salem number

Notes

1. See Properties.
2. Some of the following examples come from Hardy and Wright 1972:159–160 and pp. 178–179
3. Also Liouville's theorem can be used to "produce as many examples of transcendental numbers as we please," cf Hardy and Wright p. 161ff
4. Hardy and Wright 1972:160 / 2008:205
5. Niven 1956, Theorem 7.5.
6. Niven 1956, Corollary 7.3.
7. Niven 1956, p. 92.

References

• Artin, Michael (1991), Algebra, Prentice Hall, ISBN 0-13-004763-5, MR 1129886
• Hardy, G.H. and Wright, E.M. 1978, 2000 (with general index) An Introduction to the Theory of Numbers: 5th Edition, Clarendon Press, Oxford UK, ISBN 0-19-853171-0
• Ireland, Kenneth; Rosen, Michael (1990), A Classical Introduction to Modern Number Theory, Graduate Texts in Mathematics, 84 (Second ed.), Berlin, New York: Springer-Verlag, ISBN 0-387-97329-X, MR 1070716
• Lang, Serge (2002), Algebra, Graduate Texts in Mathematics, 211 (Revised third ed.), New York: Springer-Verlag, ISBN 978-0-387-95385-4, MR 1878556
• Niven, Ivan 1956. Irrational Numbers, Carus Mathematical Monograph no. 11, Mathematical Association of America.
• Ore, Øystein 1948, 1988, Number Theory and Its History, Dover Publications, Inc. New York, ISBN 0-486-65620-9 (pbk.)