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A [[Field_(mathematics)|field]] F is '''algebraically closed''' if there is a solution to every [[Polynomial|polynomial]] over F in one variable. Every field is contained in an algebraically closed field and there is a minimal one, called the '''algebraic closure''' of F. This general fact uses [[Zorn's Lemma]], which is equivalent to the [[Axiom of Choice]] (AC). In specific cases AC may sometimes be avoided.

A [[Field (mathematics)|field]] F is '''algebraically closed''' if there is a solution to every [[polynomial]] over F in one variable. Every field is contained in an algebraically closed field and there is a minimal one, called the '''algebraic closure''' of F. This general fact uses [[Zorn's Lemma]], which is equivalent to the [[Axiom of Choice]] (AC). In specific cases AC may sometimes be avoided.

The definition of algebraically closed has several other equivalent forms:

The definition of algebraically closed has several other equivalent forms:

<li> For every <math>a\in F</math> and every polynomial f(x), there is a <math>b\in F</math> such that <math>f(b)=a</math>.

<li> For every <math>a\in F</math> and every polynomial f(x), there is a <math>b\in F</math> such that <math>f(b)=a</math>.

<li> The only irreducible polynomials in F[x] have degree 1.

<li> The only irreducible polynomials in F[x] have degree 1.

<li> F has no finite [[Field_extension|extensions]] other than itself.

<li> F has no finite [[Field extension|extensions]] other than itself.

<li> F is a maximal field of its [[Transcendence_degree|transcendence degree]].

<li> F is a maximal field of its [[transcendence degree]].

<li> F is the union of all its [[Finitely-generated|finitely-generated]] [[Localization|localized]] [[Ring_(mathematics)|subrings]].

<li> F is the union of all its [[finitely-generated]] [[Localization|localized]] [[Ring (mathematics)|subrings]].

<li> Every [[Algebraic_variety|algebraic variety]] defined over F has a [[point]].

<li> Every [[algebraic variety]] defined over F has a [[point]].

<li> F is not finite and the [[Automorphism|automorphism]] group of F has [[Cardinality|cardinality]] strictly greater than the cardinality of F.

<li> F is not finite and the [[automorphism]] group of F has [[cardinality]] strictly greater than the cardinality of F.

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The [[Complex_number|complex numbers]] <math>\mathbb{C}</math> are algebraically closed by the [[Fundamental_Theorem_of_Algebra|Fundamental Theorem of Algebra]], however the [[Rational_number|rational numbers]] <math>\mathbb{Q}</math> are not since x²+1 does not have a root. This same polynomial shows the [[Real_number|real numbers]] <math>\mathbb{R}</math> are also not algebraically closed, yielding as a [[Corollary|corollary]] (by the equivalence) that <math>\textrm{Aut}(\mathbb{R})</math> has [[Cardinality|cardinality]] the [[Continuum|continuum]].

The [[complex number]]s <math>\mathbb{C}</math> are algebraically closed by the [[Fundamental Theorem of Algebra]], however the [[rational number]]s <math>\mathbb{Q}</math> are not since x²+1 does not have a root. This same polynomial shows the [[real number]]s <math>\mathbb{R}</math> are also not algebraically closed, yielding as a [[corollary]] (by the equivalence) that <math>\textrm{Aut}(\mathbb{R})</math> has [[cardinality]] the [[continuum]].

The complex numbers are the algebraic closure of both the real numbers and the rational numbers.

The complex numbers are the algebraic closure of both the real numbers and the rational numbers.

[[Category:Algebra]]

[[Category:Algebra]]