# Galois field

*finite field*

A field with a finite number of elements. First considered by E. Galois [1].

The number of elements of any Galois field is a power of a prime number , which is the characteristic of this field. For any prime number and any natural number there exists a (unique up to an isomorphism) field of elements. It is denoted by or by . The field contains the field as a subfield if and only if is divisible by . In particular, any field contains the field , which is called the prime field of characteristic . The field is isomorphic to the field of residue classes of the ring of integers modulo . In any fixed algebraic closure of there exists exactly one subfield for each . The correspondence is an isomorphism between the lattice of natural numbers with respect to division and the lattice of finite algebraic extensions (in ) of with respect to inclusion. The lattice of finite algebraic extensions of any Galois field within its fixed algebraic closure is such a lattice.

The algebraic extension is simple, i.e. there exists a primitive element such that . Such an will be any root of any irreducible polynomial of degree from the ring . The number of primitive elements of the extension equals

where is the Möbius function. The additive group of the field is naturally endowed with the structure of an -dimensional vector space over . As a basis one may take . The non-zero elements of form a multiplicative group, , of order , i.e. each element of is a root of the polynomial . The group is cyclic, and its generators are the primitive roots of unity of degree , the number of which is , where is the Euler function. Each primitive root of unity of degree is a primitive element of the extension , but the converse is not true. More exactly, out of the

irreducible unitary polynomials of degree over there are polynomials of which the roots are generators of .

The set of elements of coincides with the set of roots of the polynomial in , i.e. is characterized as the subfield of elements from that are invariant with respect to the automorphism , which is known as the Frobenius automorphism. If , the extension is normal (cf. Extension of a field), and its Galois group is cyclic of order . The automorphism may be taken as the generator of .

#### References

[1] | E. Galois, "Écrits et mémoires d'E. Galois" , Gauthier-Villars (1962) |

[2] | B.L. van der Waerden, "Algebra" , 1–2 , Springer (1967–1971) (Translated from German) |

[3] | N.G. [N.G. Chebotarev] Tschebotaröw, "Grundzüge der Galois'schen Theorie" , Noordhoff (1950) (Translated from Russian) |

[4] | N. Bourbaki, "Algebra" , Elements of mathematics , 1 , Springer (1989) pp. Chapt. 1–3 (Translated from French) |

**How to Cite This Entry:**

Galois field.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Galois_field&oldid=12669