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ExampleConstruction: AI-adaptedVerification: AI-generatedSession-authored (Fable 5 assisted)precheck passaudited 2026-08-02 rests on unproved material (inherited)
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The first chip identifies the source of the statement or construction; the second identifies the source of its local proof or verification.

  • Literature-sourced: the exact statement appears in a cited source; only wording and notation differ.
  • AI-adapted: a semantically identical restatement of literature-sourced material, modulo indexing, notation, and boundary cases adopted by the library.
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These labels describe origin, not correctness: citations and verification chips remain separate evidence.

Rests on 3 statements not proved in this library, by way of the results it cites. This item cites no such statement directly; it depends on results that do. The unproved premises it inherits are Cohen's first model: an infinite Dedekind-finite set of reals, Sierpiński 1947: the generalised continuum hypothesis implies the Axiom of Choice and The continuum hypothesis and its generalisation are independent of ZFC. Each is recorded with a citation to the literature and is not established here, because the track that would prove it has not yet been developed in this library. Everything else in this proof is proved here.

Under choice, the Niemytzki plane is Tychonoff and locally metrizable but not normal, paracompact, or metrizable

Example

Assume the Axiom of Choice. On M=R×[0,)M=\mathbb R\times[0,\infty), take ordinary Euclidean disks about points of positive height and, at (a,0)(a,0), the sets consisting of (a,0)(a,0) together with an open Euclidean disk tangent to the boundary there. The resulting Niemytzki plane is Tychonoff and locally metrizable, but not normal, paracompact, or metrizable.

Facts & Assumptions

Given: The tangent-disk family described in the example and the Axiom of Choice.

[L2]

Under choice, if a closed discrete subspace DD of a normal space has dense subset EE, then 2D2E2^{|D|}\le2^{|E|} (Jones's bound: under choice, a closed discrete subspace of a normal space cannot have more subsets than a dense set has subsets).

[L3]

The projection identifies D=R×{0}D=\mathbb R\times\{0\} with R\mathbb R. The Cauchy-sequence real field is complete ordered and hence Archimedean, and RP(N)\mathbb R\approx\mathcal P(\mathbb N): the ternary Cantor construction injects binary sequences into R\mathbb R; rational cuts inject R\mathbb R into P(Q)\mathcal P(\mathbb Q); QN\mathbb Q\approx\mathbb N transports this to P(N)\mathcal P(\mathbb N); and the characteristic-function bijection together with Schröder--Bernstein closes the two injections (The Cauchy-sequence reals have the least-upper-bound property, Every complete ordered field is Archimedean, The Cantor set is exactly the set of k1ak3k\sum_{k \ge 1} a_k 3^{-k} with every ak{0,2}a_k \in \{0,2\}, and this gives a bijection with {0,1}N\{0,1\}^{\mathbb{N}}, ℚ is dense in every Archimedean ordered field, The unique embedding of ℚ into an ordered field, Q\mathbb{Q} is countably infinite, Disjoint union, cartesian product, function space and power set respect equinumerosity, and for ordinals α,β\alpha, \beta the sets αβ\alpha \sqcup \beta and α×β\alpha \times \beta carry explicit well-orders, so their cardinalities exist in ZF, The Schröder-Bernstein theorem).

[L4]

E=Q×Q>0E=\mathbb Q\times\mathbb Q_{>0} is at most countable: Q\mathbb Q is countably infinite, Q>0Q\mathbb Q_{>0}\subseteq\mathbb Q is at most countable, and a product of two at-most-countable sets is at most countable. In particular EE injects into N\mathbb N (Q\mathbb{Q} is countably infinite, Every subset of an at most countable set is at most countable, A product of two at most countable sets is at most countable, Finite, countably infinite, countable, uncountable).

[L5]

There is no bijection P(N)P(P(N))\mathcal P(\mathbb N)\approx\mathcal P(\mathcal P(\mathbb N)), while injections in both directions would yield one (Cantor's theorem: AP(A)A \prec \mathcal{P}(A), The Schröder-Bernstein theorem). A paracompact Hausdorff space is normal (Every paracompact Hausdorff space is normal).

[L6]

Complete regularity separates every point from every disjoint closed set by a continuous [0,1][0,1]-valued map, and Tychonoff means complete regular plus T1T_1 (Completely regular spaces and Tychonoff (T312T_{3\frac{1}{2}}) spaces).

[L7]

Hausdorff means that distinct points have disjoint open neighbourhoods, while T1T_1 asks for an open set about each of two distinct points that misses the other (Hausdorff space: distinct points have disjoint open neighbourhoods; every metrizable space is Hausdorff and the indiscrete topology on two points is not, T0T_0 (Kolmogorov) and T1T_1 (Frechet) spaces).

Verification

technique · contradiction
1.1

Tangent disks and ordinary disks satisfy [L1]: intersections at a positive-height point contain a small ordinary disk, and a tangent disk at a boundary point contains a smaller tangent disk. Distinct points have disjoint such members: use small ordinary disks above the boundary, and for a boundary point (a,0)(a,0) choose a sufficiently small tangent disk, whose closure is tangent only at (a,0)(a,0). Thus MM is Hausdorff and hence T1T_1 by the two disjoint opens. The boundary D=R×{0}D=\mathbb R\times\{0\} is closed and discrete, while E=Q×Q>0E=\mathbb Q\times\mathbb Q_{>0} is countable and dense by the rational-density property.

L1L7
1.2

Suppose the plane were normal. Then [L2] gives an injection P(D)P(E)\mathcal P(D)\to\mathcal P(E). For the cardinal bridge in [L3], binary sequences map bijectively to the Cantor set and hence inject into R\mathbb R; x{qQ:ι(q)<x}x\mapsto\{q\in\mathbb Q:\iota(q)<x\} injects R\mathbb R into P(Q)\mathcal P(\mathbb Q) by rational density; QN\mathbb Q\approx\mathbb N transports the latter to P(N)\mathcal P(\mathbb N); and characteristic functions identify P(N)\mathcal P(\mathbb N) with binary sequences. Schröder--Bernstein therefore gives RP(N)\mathbb R\approx\mathcal P(\mathbb N), and the projection transports this to DP(N)D\approx\mathcal P(\mathbb N). By [L4], an injection ENE\to\mathbb N induces an injection P(E)P(N)\mathcal P(E)\to\mathcal P(\mathbb N), while the just-established bijection gives P(D)P(P(N))\mathcal P(D)\approx\mathcal P(\mathcal P(\mathbb N)). Their composite is therefore an injection P(P(N))P(N)\mathcal P(\mathcal P(\mathbb N))\to\mathcal P(\mathbb N). The singleton map supplies the reverse injection, so [L5] makes this impossible.

assume-contraL2L3L4L5
2.1

The tangent-disk coordinate charts obtained by radial projection from the tangency point give metrizable neighbourhoods at boundary points; Euclidean disks do so above the boundary. To separate a point pp from a closed FF not containing it, first take a basic neighbourhood of pp disjoint from FF. If p=(a,0)p=(a,0), choose a tangent disk Tr(p)T_r(p) disjoint from FF and define f(p)=1f(p)=1 and, for (u,v)(u,v) with v>0v>0, f(u,v)=max ⁣{0,1(ua)2+v2rv}.f(u,v)=\max\!\left\{0,1-\frac{(u-a)^2+v^2}{rv}\right\}. Its support is the smaller tangent disk Tr/2(p)T_{r/2}(p), and f1f\to1 at pp because (ua)2+v2<εrv(u-a)^2+v^2<\varepsilon rv is exactly membership in a sufficiently small tangent disk. It is ordinarily continuous above the boundary and zero on a tangent neighbourhood of every other boundary point, so it is continuous on MM and vanishes on FF. If pp has positive height, an ordinary Euclidean bump supported in a small disk disjoint from FF and from the boundary has the same properties, extended by zero elsewhere. Thus MM is completely regular; with the T1T_1 conclusion of step 1.1, [L6] makes it Tychonoff and locally metrizable.

L6step 1.1
3.1

Hence the plane is not normal. If it were paracompact, its Tychonoff property gives Hausdorffness and [L5] would make it normal; if it were metrizable, Stone's theorem, under choice: every metric space is paracompact would make it paracompact. Both are impossible.

L5step 2.1step 1.2
4.1

This proves the stated profile.

step 2.1step 3.1discharge-contradiction

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