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Relative consistency of DC with failure of Stone's theorem
Statement
If is consistent, then is consistent with the failure of Stone's theorem: there is a model of (The axiom of dependent choice: a relation in which every element is related to something admits an -indexed chain) containing a metric space that is not paracompact (Paracompactness: every open cover has a locally finite open refinement, with no separation axiom built into the word, Refinements, locally finite families, point-finite families, and star refinements). The witness space of this separation is not zero-dimensional: it is the metric sum of the nondegenerate connected components of fact [F4], so no base of clopen sets can exist. (The source's zero-dimensional witness is a different space, the rational-parameter subset , whose nonparacompactness is proved there in without DC.)
Facts & Assumptions
Given: The regular- symmetric model of The Good-Tree-Watson symmetric Stone model with , its components , and the assumed consistency of .
In the transitive-ground presentation of the construction, the model is a transitive model of ZF between the ground model and the full extension, and it is closed under -sequences from the full extension (Hereditarily symmetric interpretations form a transitive ZF model, The Good-Tree-Watson symmetric model is closed under omega-sequences from the full extension, Forcing theorem).
Dependent choice holds in : given a relation on a set of that is entire there, for each prescribed starting point in the set, DC in the full extension supplies an -sequence of -related points starting at , and by [F1] that sequence lies in . (The axiom of dependent choice: a relation in which every element is related to something admits an -indexed chain)
No function of chooses a nonempty proper subset of every component (The symmetric Stone model has no componentwise proper selector).
The metric sum carries the metric that agrees with the metric of each component and puts distance between different components; its topology is the topological sum of the components, each is a clopen connected subspace with more than one point, and the whole space is metrizable (Metric space: iff , symmetry, and the triangle inequality; pseudometric and ultrametric, Open ball, closed ball and sphere in a metric space, The metric topology: a set is open when every one of its points has a ball around it inside the set; closed means open complement, Arbitrary unions and finite intersections of open sets are open, open balls are open and closed balls are closed, Topology on a set, open and closed sets, clopen sets, the closed-set axiomatisation, and the coarser/finer comparison). Because a clopen connected set with more than one point has no proper nonempty clopen subset, the space has no base of clopen sets and is therefore not zero-dimensional.
Good--Tree--Watson's Theorem 2 explicitly states, relative to ZF, the consistency of a model with a metrizable nonparacompact space, and their Theorem 3 explicitly states that Stone's theorem is not provable in ZF+DC. In the generalization immediately following Theorem 3 they replace the finite-support construction by the regular- construction, take , and use closure under countable sequences to obtain DC. Thus the relative-consistency bridge used here is the published theorem itself, not an inference from the existence of a transitive ground model. [source]
Proof technique: direct.
Proof
Invoke the external relative-consistency construction of [F5], in its regular- form with , and write for its symmetric model. The invocation is exactly the source's relative-consistency theorem; it does not infer a transitive ground model from .
The model satisfies DC by [F2], because every -sequence in the full extension with values in is already in by [F1].
In form the metric sum with the metric of [F4]; it is a metrizable space whose components are clopen and connected.
Let . This is an open cover. Every member lies in one component, since distinct components have distance , and is a proper subset of that component: under , a radius- ball corresponds to a bounded ordinary interval.
Suppose that has a locally finite open refining cover , and define Each is nonempty. Indeed, a point of has a nonempty open neighbourhood meeting only finitely many refinement members . Starting with , recursively choose the nonempty open set if that intersection is nonempty, and otherwise choose . The latter is nonempty: an open disjoint from the open set cannot be contained in . Thus every point of avoids the boundaries of the , and it avoids the boundary of every other refinement member because that member misses . Hence . The set is also proper. Choose a nonempty meeting . Such a member exists because covers . By refinement and step 3.1, lies in and is contained in a proper ball there. It is therefore a nonempty proper open subset of the connected space , so is nonempty and disjoint from . Consequently is a function assigning a nonempty proper subset to every component.
By [F3] no such function exists in ; hence has no locally finite open refining cover, and is not paracompact.
Steps 2.1 and 5.1 identify the DC and nonparacompactness conclusions inside the published model. By [F5], that construction has exactly the asserted consistency strength relative to ZF. The argument uses the regular- presentation and not the finite-support -indexed one.
Depends on
- The Good-Tree-Watson symmetric Stone model
- The Good-Tree-Watson symmetric model is closed under omega-sequences from the full extension
- The symmetric Stone model has no componentwise proper selector
- The axiom of dependent choice: a relation in which every element is related to something admits an $\mathbb{N}$-indexed chain
- Paracompactness: every open cover has a locally finite open refinement, with no separation axiom built into the word
- Refinements, locally finite families, point-finite families, and star refinements
- Metric space: $d(x,y) = 0$ iff $x = y$, symmetry, and the triangle inequality; pseudometric and ultrametric
- Open ball, closed ball and sphere in a metric space
- The metric topology: a set is open when every one of its points has a ball around it inside the set; closed means open complement
- Arbitrary unions and finite intersections of open sets are open, open balls are open and closed balls are closed
- Topology on a set, open and closed sets, clopen sets, the closed-set axiomatisation, and the coarser/finer comparison
- Forcing theorem
- Hereditarily symmetric interpretations form a transitive ZF model
Used by
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Sources
- C. Good, I. J. Tree, and W. S. Watson, On Stone's theorem and the axiom of choice (standard reference, not scraped)
- Thomas J. Jech, The Axiom of Choice (standard reference, not scraped)