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Field theory and polynomials Definition

Galois connection between subgroups and subfields

Builds on Field extensionIntermediate fieldFixed field

Definition

Let L/KL/K be a field extension, and let G=Aut⁡(L/K)G = \Aut(L/K) be the group of field automorphisms of LL that fix every element of KK. Let 𝒮\mathcal{S} be the set of subgroups of GG and let ℱ\mathcal{F} be the set of intermediate fields of L/KL/K, both ordered by inclusion. For H∈𝒮H \in \mathcal{S}, let LH={x∈L:σ(x)=x for all σ∈H}L^H = \set{x \in L : \sigma(x) = x \text{ for all } \sigma \in H} be the fixed field of HH. For E∈ℱE \in \mathcal{F}, let Aut⁡(L/E)={σ∈G:σ(x)=x for all x∈E}\Aut(L/E) = \set{\sigma \in G : \sigma(x) = x \text{ for all } x \in E}. The Galois connection between subgroups and subfields of L/KL/K is the pair of maps 𝒮→ℱ, H↦LHandℱ→𝒮, E↦Aut⁡(L/E).\mathcal{S} \to \mathcal{F},\ H \mapsto L^H \qquad \text{and} \qquad \mathcal{F} \to \mathcal{S},\ E \mapsto \Aut(L/E). It is an antitone Galois connection: for all H∈𝒮H \in \mathcal{S} and E∈ℱE \in \mathcal{F}, E⊆LH  ⟺  H⊆Aut⁡(L/E).E \subseteq L^H \iff H \subseteq \Aut(L/E).

Remarks

Both sides of the equivalence say that every element of HH fixes every element of EE, which is the check. From the equivalence, H⊆Aut⁡(L/LH)H \subseteq \Aut(L/L^H) and E⊆LAut⁡(L/E)E \subseteq L^{\Aut(L/E)}, and each map reverses inclusions. Example: for L=ℚ(2,3)L = \Q(\sqrt{2}, \sqrt{3}) over ℚ\Q, the group GG has four elements, and the maps are inverse bijections between its five subgroups and the five intermediate fields. Non-example of a bijection: for L=ℚ(23)L = \Q(\sqrt[3]{2}) over ℚ\Q, the group GG is trivial, because 23\sqrt[3]{2} is the only cube root of 22 in the real field LL. Then the map E↦Aut⁡(L/E)E \mapsto \Aut(L/E) sends both ℚ\Q and LL to the trivial group, so it is not injective. The maps are inverse bijections when L/KL/K is a finite Galois extension, by the fundamental theorem of Galois theory.

Reading path

To understand this page from the ground up, read these 13 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. Group concept
  2. Ring definition
  3. Commutative ring definition
  4. Ring homomorphism definition
  5. Subring definition
  6. Unit of a ring definition
  7. Ring isomorphism definition
  8. Field definition
  9. Subfield definition
  10. Fixed field definition
  11. Field extension definition
  12. Intermediate field definition
  13. Galois connection between subgroups and subfields definition

Further reading

  1. Web https://en.wikipedia.org/wiki/Galois_connection
About this page
TagFW68FVNKMJ
URIm2://FW68FVNKMJ
Kinddefinition
WritingW0 stub
MSC12F10, 06A15
DefinesGalois connection between subgroups and subfields
UsesField extension, Intermediate field, Fixed field
Changed2 November 2026

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