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Mathlib.Topology.Algebra.Field

Topological fields #

A topological division ring is a topological ring whose inversion function is continuous at every non-zero element.

Left-multiplication by a nonzero element of a topological division ring is proper, i.e., inverse images of compact sets are compact.

Right-multiplication by a nonzero element of a topological division ring is proper, i.e., inverse images of compact sets are compact.

    A topological division ring is a division ring with a topology where all operations are continuous, including inversion.

    Instances

      The (topological-space) closure of a subfield of a topological field is itself a subfield.

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        This section is about affine homeomorphisms from a topological field 𝕜 to itself. Technically it does not require 𝕜 to be a topological field, a topological ring that happens to be a field is enough.

        @[simp]
        theorem affineHomeomorph_symm_apply {𝕜 : Type u_2} [Field 𝕜] [TopologicalSpace 𝕜] [TopologicalRing 𝕜] (a : 𝕜) (b : 𝕜) (h : a 0) (y : 𝕜) :
        ↑(Homeomorph.symm (affineHomeomorph a b h)) y = (y - b) / a
        @[simp]
        theorem affineHomeomorph_apply {𝕜 : Type u_2} [Field 𝕜] [TopologicalSpace 𝕜] [TopologicalRing 𝕜] (a : 𝕜) (b : 𝕜) (h : a 0) (x : 𝕜) :
        ↑(affineHomeomorph a b h) x = a * x + b
        def affineHomeomorph {𝕜 : Type u_2} [Field 𝕜] [TopologicalSpace 𝕜] [TopologicalRing 𝕜] (a : 𝕜) (b : 𝕜) (h : a 0) :
        𝕜 ≃ₜ 𝕜

        The map fun x => a * x + b, as a homeomorphism from 𝕜 (a topological field) to itself, when a ≠ 0.

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          theorem IsLocalMin.inv {α : Type u_2} {β : Type u_3} [TopologicalSpace α] [LinearOrderedSemifield β] {f : αβ} {a : α} (h1 : IsLocalMin f a) (h2 : ∀ᶠ (z : α) in nhds a, 0 < f z) :

          Some results about functions on preconnected sets valued in a ring or field with a topology.

          theorem IsPreconnected.eq_one_or_eq_neg_one_of_sq_eq {α : Type u_2} {𝕜 : Type u_3} {f : α𝕜} {S : Set α} [TopologicalSpace α] [TopologicalSpace 𝕜] [T1Space 𝕜] [Ring 𝕜] [NoZeroDivisors 𝕜] (hS : IsPreconnected S) (hf : ContinuousOn f S) (hsq : Set.EqOn (f ^ 2) 1 S) :
          Set.EqOn f 1 S Set.EqOn f (-1) S

          If f is a function α → 𝕜 which is continuous on a preconnected set S, and f ^ 2 = 1 on S, then either f = 1 on S, or f = -1 on S.

          theorem IsPreconnected.eq_or_eq_neg_of_sq_eq {α : Type u_2} {𝕜 : Type u_3} {f : α𝕜} {g : α𝕜} {S : Set α} [TopologicalSpace α] [TopologicalSpace 𝕜] [T1Space 𝕜] [Field 𝕜] [HasContinuousInv₀ 𝕜] [ContinuousMul 𝕜] (hS : IsPreconnected S) (hf : ContinuousOn f S) (hg : ContinuousOn g S) (hsq : Set.EqOn (f ^ 2) (g ^ 2) S) (hg_ne : ∀ {x : α}, x Sg x 0) :
          Set.EqOn f g S Set.EqOn f (-g) S

          If f, g are functions α → 𝕜, both continuous on a preconnected set S, with f ^ 2 = g ^ 2 on S, and g z ≠ 0 all z ∈ S, then either f = g or f = -g on S.

          theorem IsPreconnected.eq_of_sq_eq {α : Type u_2} {𝕜 : Type u_3} {f : α𝕜} {g : α𝕜} {S : Set α} [TopologicalSpace α] [TopologicalSpace 𝕜] [T1Space 𝕜] [Field 𝕜] [HasContinuousInv₀ 𝕜] [ContinuousMul 𝕜] (hS : IsPreconnected S) (hf : ContinuousOn f S) (hg : ContinuousOn g S) (hsq : Set.EqOn (f ^ 2) (g ^ 2) S) (hg_ne : ∀ {x : α}, x Sg x 0) {y : α} (hy : y S) (hy' : f y = g y) :
          Set.EqOn f g S

          If f, g are functions α → 𝕜, both continuous on a preconnected set S, with f ^ 2 = g ^ 2 on S, and g z ≠ 0 all z ∈ S, then as soon as f = g holds at one point of S it holds for all points.