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ExampleConstruction: AI-adaptedVerification: AI-generatedSession-authored (Fable 5 assisted)precheck passverified 2026-08-04 (gpt-5.6-sol-codex-subscription)
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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.

The discrete, indiscrete, cofinite, cocountable, particular-point and Sierpinski topologies placed in the connectedness hierarchy

Example

Let XX be a set carrying one of the six topologies of The discrete, indiscrete, cofinite, cocountable, particular-point and Sierpinski topologies. The table records where each sits, with connectedness as in Separation of a topological space, connected and disconnected spaces, clopen sets, and connected subsets, local connectedness as in Locally connected and locally path-connected spaces: a neighbourhood base of open connected, respectively open path-connected, sets at every point and path-connectedness as in Paths, path-connected spaces and path components.

topologyconnectedlocally connectedpath-connectedcomponents
discreteonly if XX has at most one pointyesonly if XX has at most one pointthe singletons
indiscreteyesyesyesXX
cofinite, XX infiniteyesyesnot decided hereXX
cocountable, XX uncountableyesyesnot decided hereXX
particular point ppyesyesyesXX
SierpinskiyesyesyesSS

Sierpinski space is the particular-point topology on a two-point set (The discrete, indiscrete, cofinite, cocountable, particular-point and Sierpinski topologies), so its row is an instance of the row above it and is not verified separately.

The component column reads at nonempty XX; the empty space is connected and has no components at all, there being no points (The components of a space are its maximal connected subsets, they partition it, and each of them is closed).

Two entries are deliberately left open. No item among this page's declared prerequisites settles whether the cofinite topology on an infinite XX, or the cocountable topology on an uncountable XX, is path-connected, so the table says nothing either way.

Facts & Assumptions

Given: A set XX carrying one of the six standard topologies.

[A2]

The open sets are: all subsets (discrete); \varnothing and XX (indiscrete); \varnothing and the sets of finite complement (cofinite); \varnothing and the sets of at most countable complement (cocountable); \varnothing and the sets containing pp (particular point). A union of two finite sets is finite (The discrete, indiscrete, cofinite, cocountable, particular-point and Sierpinski topologies, Finite, countably infinite, countable, uncountable).

[A3]

XX is locally connected when every open UU and every xUx \in U admit an open connected VV with xVUx \in V \subseteq U; a component of XX is the largest connected set through a point, and the components partition XX (Locally connected and locally path-connected spaces: a neighbourhood base of open connected, respectively open path-connected, sets at every point, Connected components, quasicomponents, and totally disconnected spaces, The components of a space are its maximal connected subsets, they partition it, and each of them is closed).

[A5]

A nonempty set is at most countable exactly when some surjection N\mathbb{N} \to it exists; there is a bijection NN×N\mathbb{N} \to \mathbb{N} \times \mathbb{N}; R\mathbb{R} is uncountable (A nonempty set is at most countable iff it is a surjective image of N\mathbb{N}, N×NN\mathbb{N} \times \mathbb{N} \approx \mathbb{N}, R\mathbb{R} is uncountable (Cantor's nested intervals, 1874), Finite, countably infinite, countable, uncountable).

Verification

technique · direct
1.1

Discrete. If XX has two distinct points x,yx, y then ({x},X{x})(\{x\}, X \setminus \{x\}) is a separation by [A2] and [A1], so XX is disconnected; every connected subset therefore has at most one point, the subspace topology on a subset again being discrete, so the components are the singletons by [A3], and XX is not path-connected, a path being in particular a connected image. Every singleton is open and connected, so XX is locally connected by [A3].

A1A2A3
1.2

Indiscrete. The only open sets are \varnothing and XX by [A2], so no two nonempty open sets are disjoint and XX is connected by [A1]; hence, for nonempty XX, its only component is XX by [A3], and XX itself is an open connected set containing every point, so XX is locally connected by [A3].

A1A2A3
1.3

Indiscrete, path-connectedness. Every function γ:[0,1]X\gamma : [0,1] \to X is continuous by [A4], the preimages of \varnothing and XX being \varnothing and [0,1][0,1]; so for x,yXx, y \in X the function with γ(0)=x\gamma(0) = x and γ(t)=y\gamma(t) = y for t>0t > 0 is a path from xx to yy, and XX is path-connected.

A2A4
1.4

Cofinite, XX infinite. Let U,VU, V be nonempty open sets, with complements CUC_U, CVC_V finite by [A2]. Then X(UV)=CUCVX \setminus (U \cap V) = C_U \cup C_V is finite by [A2], so UVU \cap V \ne \varnothing, XX being infinite. Hence no separation exists and XX is connected by [A1].

A1A2
1.5

Cocountable, XX uncountable, and XX a subset of R\mathbb{R} or any uncountable set. Suppose (U,V)(U,V) were a separation. By [A2] the complements of UU and of VV are at most countable, and those complements are VV and UU respectively, so both UU and VV are nonempty and at most countable with UV=XU \cup V = X.

A1A2
1.6

Particular point pp. Every nonempty open set contains pp by [A2], so two nonempty open sets meet and XX is connected by [A1]; its only component is XX by [A3].

A1A2A3
2.1

In the situation of step 1.5, [A5] gives surjections f,g:NUf, g : \mathbb{N} \to U and NV\mathbb{N} \to V, and h:N×NXh : \mathbb{N} \times \mathbb{N} \to X with h(0,k):=f(k)h(0,k) := f(k) and h(m,k):=g(k)h(m,k) := g(k) for m0m \ne 0 is onto UV=XU \cup V = X; composing with a bijection NN×N\mathbb{N} \to \mathbb{N} \times \mathbb{N} from [A5] shows XX at most countable, contrary to hypothesis. So the cocountable topology on an uncountable XX is connected.

step 1.5A5
2.2

Particular point is locally connected and path-connected. Every nonempty open UU contains pp and carries as a subspace the particular-point topology on UU with the same pp, hence is connected by step 1.6, so [A3] is witnessed by UU itself. For x,yXx, y \in X define γ(0):=x\gamma(0) := x, γ(1):=y\gamma(1) := y and γ(t):=p\gamma(t) := p for 0<t<10 < t < 1; the preimage of an open VV is \varnothing if V=V = \varnothing, and otherwise contains (0,1)(0,1) and is one of (0,1)(0,1), [0,1)[0,1), (0,1](0,1], [0,1][0,1], all open in [0,1][0,1] by [A6]. So γ\gamma is continuous by [A4] and is a path from xx to yy.

step 1.6A2A3A4A6
3.1

Cofinite and cocountable are locally connected. A nonempty open UU carries as a subspace the cofinite, respectively cocountable, topology on UU by [A2] and [A1]; UU is infinite, respectively uncountable, its complement being finite, respectively at most countable, while XX is not. So UU is connected by step 1.4, respectively step 2.1, and being open and containing each of its points it witnesses [A3].

step 1.4step 2.1A1A2A3
4.1

Sierpinski space is the particular-point topology on a two-point set with particular point its open point, by [A2], so steps 1.6 and 2.2 apply to it verbatim; and every nonempty connected space has its whole underlying set as its unique component by [A3]. This completes the table.

step 1.1step 1.2step 1.3step 1.4step 2.1step 2.2step 3.1A2A3

Remarks

  • Connectedness is cheap when there are few open sets. Four of the six topologies are connected for the same structural reason: no two nonempty open sets are disjoint. That is immediate for the indiscrete and particular-point topologies, and for the cofinite and cocountable ones it is the statement that the ambient set is not a union of two small sets.

  • Local connectedness here is never informative. In every connected case above, every nonempty open subspace is again of the same kind and hence connected, so local connectedness holds for free. A space where local connectedness carries information must have open sets that are not themselves connected, which is what happens in R2\mathbb{R}^2 and its subspaces.

  • Why two cells are left blank. A path in the cofinite or cocountable topology is a continuous map out of [0,1][0,1], and deciding whether a non-constant one exists needs machinery this page's declared prerequisites do not supply: for the cofinite case a comparison of X|X| with the cardinality of R\mathbb{R}, and for the cocountable case an argument about the image of a dense countable subset. Neither is available here, so the honest entry is that the question is not settled.

Depends on

Used by

Nothing in the library uses this result yet.

Dependency tree · next 3 levels

Direct dependencies and their dependencies through the next three levels: 122 results over 26 levels. An arrow runs from a result to what uses it, and this result sits at the bottom with a heavier outline. Click the chart to enlarge it.

Sources