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Field theory and polynomials Definition

Pseudo algebraically closed field

Builds on FieldAlgebraic closureAlgebraically closed fieldFinite field

Definition

Let KK be a field with algebraic closure Kˉ\bar K. An affine variety V⊆KˉnV \subseteq \bar K^n defined over KK is the set of common zeros in Kˉn\bar K^n of a set of polynomials in K[X1,…,Xn]K[X_1, \dots, X_n]. It is absolutely irreducible if it is irreducible as a variety over Kˉ\bar K. This means that it is not the union of two proper closed subsets defined over Kˉ\bar K. A point of VV is KK-rational if all its coordinates lie in KK. The field KK is pseudo algebraically closed (PAC) if every nonempty absolutely irreducible affine variety defined over KK has a KK-rational point.

Remarks

It is equivalent to ask the condition only for curves, that is, for varieties of dimension 11.

Example: every algebraically closed field KK is PAC, since then Kˉ=K\bar K = K and every point of a nonempty variety is KK-rational. Every infinite algebraic extension of a finite field is PAC.

Non-example: ℚ\Q and ℝ\R are not PAC. The curve VV defined by x2+y2+1=0x^2 + y^2 + 1 = 0 is absolutely irreducible, since x2+y2+1x^2 + y^2 + 1 is irreducible over ℂ\C, and VV is nonempty, since it contains (i,0)(i, 0). But it has no point with real coordinates, because x2+y2+1>0x^2 + y^2 + 1 > 0 for real x,yx, y.

Non-example: a finite field 𝔽q\F_q is not PAC. For instance, the curve yq−y=xq+1−x2+1y^q - y = x^{q+1} - x^2 + 1 is absolutely irreducible, because the degree q+1q + 1 of the right side is prime to the characteristic of 𝔽q\F_q. It has no 𝔽q\F_q-rational point, because yq−y=0y^q - y = 0 for y∈𝔽qy \in \F_q, while xq+1−x2+1=1x^{q+1} - x^2 + 1 = 1 for x∈𝔽qx \in \F_q.

Reading path

To understand this page from the ground up, read these 5 pages in order. The foundations come first.

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Each page uses the pages above it. The colour of the bar on a box shows how strong the evidence is, and the box also gives it as text. The list has the same pages.
  • no evidence level
  1. Algebraic closure concept
  2. Algebraically closed field concept
  3. Field concept
  4. Finite field concept
  5. Pseudo algebraically closed field definition

Further reading

  1. Web https://en.wikipedia.org/wiki/Pseudo_algebraically_closed_field
About this page
TagP32FQE1H0D
URIm2://P32FQE1H0D
Kinddefinition
WritingW0 stub
MSC12E30, 12L12, 14G05
DefinesPseudo algebraically closed field
UsesField, Algebraic closure, Algebraically closed field, Finite field
Changed2 November 2026

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