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TheoremStatement: Literature-sourcedProof: AI-adaptedPipeline-generatedaudited 2026-09-22
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Relative consistency of DC with failure of Stone's theorem

Statement

If ZF is consistent, then ZF+DC is consistent with the failure of Stone's theorem: there is a model of ZF+DC (The axiom of dependent choice: a relation in which every element is related to something admits an N-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 nQn, whose nonparacompactness is proved there in ZF without DC.)

Facts & Assumptions

Given: The regular-λ symmetric model of The Good-Tree-Watson symmetric Stone model with λ=ω1, its components Rξ, and the assumed consistency of ZF.

[F1]

In the transitive-ground presentation of the construction, the model N is a transitive model of ZF between the ground model and the full extension, and it is closed under ω-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).

[F2]

Dependent choice holds in N: given a relation R on a set of N that is entire there, for each prescribed starting point a in the set, DC in the full extension supplies an ω-sequence of R-related points starting at a, and by [F1] that sequence lies in N. (The axiom of dependent choice: a relation in which every element is related to something admits an N-indexed chain)

[F3]

No function of N chooses a nonempty proper subset of every component Rξ (The symmetric Stone model has no componentwise proper selector).

[F4]

The metric sum ξ<λRξ carries the metric that agrees with the metric of each component and puts distance 1 between different components; its topology is the topological sum of the components, each Rξ is a clopen connected subspace with more than one point, and the whole space is metrizable (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). 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.

[F5]

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-λ construction, take λ=ω1, 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

1.1

Invoke the external relative-consistency construction of [F5], in its regular-λ form with λ=ω1, and write N for its symmetric model. The invocation is exactly the source's relative-consistency theorem; it does not infer a transitive ground model from Con(ZF).

givenF5
2.1

The model satisfies DC by [F2], because every ω-sequence in the full extension with values in N is already in N by [F1].

step 1.1F1F2
2.2

In N form the metric sum X:=ξ<λRξ with the metric of [F4]; it is a metrizable space whose components Rξ are clopen and connected.

step 1.1F4
3.1

Let U={BX(x,1/3):xX}. This is an open cover. Every member lies in one component, since distinct components have distance 1, and is a proper subset of that component: under d(r,s)=rs/(1+rs), a radius-1/3 ball corresponds to a bounded ordinary interval.

step 2.2F4
4.1

Suppose that U has a locally finite open refining cover V, and define S=XVV(VV),Sξ=SRξ. Each Sξ is nonempty. Indeed, a point of Rξ has a nonempty open neighbourhood WRξ meeting only finitely many refinement members V0,,Vk. Starting with W0=W, recursively choose the nonempty open set Wi+1=WiVi if that intersection is nonempty, and otherwise choose Wi+1=WiVi. The latter is nonempty: an open Wi disjoint from the open set Vi cannot be contained in Vi. Thus every point of Wk+1 avoids the boundaries of the Vi, and it avoids the boundary of every other refinement member because that member misses W. Hence Wk+1Sξ. The set Sξ is also proper. Choose a nonempty VV meeting Rξ. Such a member exists because V covers Rξ. By refinement and step 3.1, V lies in Rξ and is contained in a proper ball there. It is therefore a nonempty proper open subset of the connected space Rξ, so VV is nonempty and disjoint from Sξ. Consequently ξSξ is a function assigning a nonempty proper subset to every component.

step 3.1F4
5.1

By [F3] no such function exists in N; hence U has no locally finite open refining cover, and X is not paracompact.

step 4.1F3
6.1

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-λ presentation and not the finite-support ω-indexed one.

step 1.1step 2.1step 5.1F5

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