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Under choice, the Niemytzki plane is Tychonoff and locally metrizable but not normal, paracompact, or metrizable
Example
Assume the Axiom of Choice. On , take ordinary Euclidean disks about points of positive height and, at , the sets consisting of 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.
A family covering every point and admitting a contained member around each point of an intersection is a basis (A family is a basis for a unique topology iff it covers the set and every point of an intersection of two members lies in a member inside that intersection; finite intersections of any subbasis form a basis).
Under choice, if a closed discrete subspace of a normal space has dense subset , then (Jones's bound: under choice, a closed discrete subspace of a normal space cannot have more subsets than a dense set has subsets).
The projection identifies with . The Cauchy-sequence real field is complete ordered and hence Archimedean, and : the ternary Cantor construction injects binary sequences into ; rational cuts inject into ; transports this to ; 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 with every , and this gives a bijection with , ℚ is dense in every Archimedean ordered field, The unique embedding of ℚ into an ordered field, is countably infinite, Disjoint union, cartesian product, function space and power set respect equinumerosity, and for ordinals the sets and carry explicit well-orders, so their cardinalities exist in ZF, The Schröder-Bernstein theorem).
is at most countable: is countably infinite, is at most countable, and a product of two at-most-countable sets is at most countable. In particular injects into ( 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).
There is no bijection , while injections in both directions would yield one (Cantor's theorem: , The Schröder-Bernstein theorem). A paracompact Hausdorff space is normal (Every paracompact Hausdorff space is normal).
Complete regularity separates every point from every disjoint closed set by a continuous -valued map, and Tychonoff means complete regular plus (Completely regular spaces and Tychonoff () spaces).
Hausdorff means that distinct points have disjoint open neighbourhoods, while 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, (Kolmogorov) and (Frechet) spaces).
Verification
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 choose a sufficiently small tangent disk, whose closure is tangent only at . Thus is Hausdorff and hence by the two disjoint opens. The boundary is closed and discrete, while is countable and dense by the rational-density property.
Suppose the plane were normal. Then [L2] gives an injection . For the cardinal bridge in [L3], binary sequences map bijectively to the Cantor set and hence inject into ; injects into by rational density; transports the latter to ; and characteristic functions identify with binary sequences. Schröder--Bernstein therefore gives , and the projection transports this to . By [L4], an injection induces an injection , while the just-established bijection gives . Their composite is therefore an injection . The singleton map supplies the reverse injection, so [L5] makes this impossible.
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 from a closed not containing it, first take a basic neighbourhood of disjoint from . If , choose a tangent disk disjoint from and define and, for with , Its support is the smaller tangent disk , and at because 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 and vanishes on . If has positive height, an ordinary Euclidean bump supported in a small disk disjoint from and from the boundary has the same properties, extended by zero elsewhere. Thus is completely regular; with the conclusion of step 1.1, [L6] makes it Tychonoff and locally metrizable.
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.
This proves the stated profile.
Depends on
- A locally metrizable space: every point has a metrizable open neighbourhood
- Completely regular spaces and Tychonoff ($T_{3\frac{1}{2}}$) spaces
- Hausdorff space: distinct points have disjoint open neighbourhoods; every metrizable space is Hausdorff and the indiscrete topology on two points is not
- $T_0$ (Kolmogorov) and $T_1$ (Frechet) spaces
- A family is a basis for a unique topology iff it covers the set and every point of an intersection of two members lies in a member inside that intersection; finite intersections of any subbasis form a basis
- Basis and subbasis for a topology, and the topology generated by a family of sets
- Jones's bound: under choice, a closed discrete subspace of a normal space cannot have more subsets than a dense set has subsets
- The Cauchy-sequence reals have the least-upper-bound property
- Every complete ordered field is Archimedean
- The Cantor set is exactly the set of $\sum_{k \ge 1} a_k 3^{-k}$ with every $a_k \in \{0,2\}$, and this gives a bijection with $\{0,1\}^{\mathbb{N}}$
- $\mathbb{Q}$ is countably infinite
- ℚ is dense in every Archimedean ordered field
- The unique embedding of ℚ into an ordered field
- Disjoint union, cartesian product, function space and power set respect equinumerosity, and for ordinals $\alpha, \beta$ the sets $\alpha \sqcup \beta$ and $\alpha \times \beta$ carry explicit well-orders, so their cardinalities exist in ZF
- A product of two at most countable sets is at most countable
- Every subset of an at most countable set is at most countable
- Finite, countably infinite, countable, uncountable
- Cantor's theorem: $A \prec \mathcal{P}(A)$
- The Schröder-Bernstein theorem
- Stone's theorem, under choice: every metric space is paracompact
- Every paracompact Hausdorff space is normal
- The Axiom of Choice
Used by
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Sources
- D. Chodounský, Non-normality and relative normality of Niemytzki plane (standard reference, not scraped)