How statement and proof provenance work
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.
- AI-generated: a genuinely novel statement formulated by AI, with no source for the claim itself.
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 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.
| topology | connected | locally connected | path-connected | components |
|---|---|---|---|---|
| discrete | only if has at most one point | yes | only if has at most one point | the singletons |
| indiscrete | yes | yes | yes | |
| cofinite, infinite | yes | yes | not decided here | |
| cocountable, uncountable | yes | yes | not decided here | |
| particular point | yes | yes | yes | |
| Sierpinski | yes | yes | yes |
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 ; 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 , or the cocountable topology on an uncountable , is path-connected, so the table says nothing either way.
Facts & Assumptions
Given: A set carrying one of the six standard topologies.
A separation of a space is a pair of open, nonempty, disjoint sets covering it; a space is connected when none exists, and a subset is connected when it is connected as a subspace (Separation of a topological space, connected and disconnected spaces, clopen sets, and connected subsets, Subspace topology: the traces of the open sets, its closed sets and its bases, the continuity of the inclusion, and the characteristic property of a map into a subspace, Topology on a set, open and closed sets, clopen sets, the closed-set axiomatisation, and the coarser/finer comparison). If is path-connected then is connected, and the same holds for a subset (Every path-connected space is connected, and every path component lies inside a component, claim 2).
The open sets are: all subsets (discrete); and (indiscrete); and the sets of finite complement (cofinite); and the sets of at most countable complement (cocountable); and the sets containing (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).
is locally connected when every open and every admit an open connected with ; a component of is the largest connected set through a point, and the components partition (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).
is continuous at every point exactly when is open for every open in the codomain (For a map of spaces the following agree: continuity at every point, preimages of open sets open, preimages of closed sets closed, preimages of subbasic open sets open, and , claim (a) iff (b), Continuity of a map of topological spaces at a point and globally); and a space is connected exactly when every continuous map to the two-point discrete space is constant (For a topological space the following agree: no separation exists, the only clopen subsets are and , and every continuous map to the two-point discrete space is constant, claims 1 and 2).
A nonempty set is at most countable exactly when some surjection it exists; there is a bijection ; is uncountable (A nonempty set is at most countable iff it is a surjective image of , , is uncountable (Cantor's nested intervals, 1874), Finite, countably infinite, countable, uncountable).
carries the subspace topology from , in which , , and are open (Intervals of : the nine order-convex forms, nondegeneracy, and length, Subspace topology: the traces of the open sets, its closed sets and its bases, the continuity of the inclusion, and the characteristic property of a map into a subspace, Paths, path-connected spaces and path components).
Verification
Discrete. If has two distinct points then is a separation by [A2] and [A1], so 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 is not path-connected, a path being in particular a connected image. Every singleton is open and connected, so is locally connected by [A3].
Indiscrete. The only open sets are and by [A2], so no two nonempty open sets are disjoint and is connected by [A1]; hence, for nonempty , its only component is by [A3], and itself is an open connected set containing every point, so is locally connected by [A3].
Indiscrete, path-connectedness. Every function is continuous by [A4], the preimages of and being and ; so for the function with and for is a path from to , and is path-connected.
Cofinite, infinite. Let be nonempty open sets, with complements , finite by [A2]. Then is finite by [A2], so , being infinite. Hence no separation exists and is connected by [A1].
Cocountable, uncountable, and a subset of or any uncountable set. Suppose were a separation. By [A2] the complements of and of are at most countable, and those complements are and respectively, so both and are nonempty and at most countable with .
Particular point . Every nonempty open set contains by [A2], so two nonempty open sets meet and is connected by [A1]; its only component is by [A3].
In the situation of step 1.5, [A5] gives surjections and , and with and for is onto ; composing with a bijection from [A5] shows at most countable, contrary to hypothesis. So the cocountable topology on an uncountable is connected.
Particular point is locally connected and path-connected. Every nonempty open contains and carries as a subspace the particular-point topology on with the same , hence is connected by step 1.6, so [A3] is witnessed by itself. For define , and for ; the preimage of an open is if , and otherwise contains and is one of , , , , all open in by [A6]. So is continuous by [A4] and is a path from to .
Cofinite and cocountable are locally connected. A nonempty open carries as a subspace the cofinite, respectively cocountable, topology on by [A2] and [A1]; is infinite, respectively uncountable, its complement being finite, respectively at most countable, while is not. So is connected by step 1.4, respectively step 2.1, and being open and containing each of its points it witnesses [A3].
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.
Remarks
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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.
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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 and its subspaces.
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Why two cells are left blank. A path in the cofinite or cocountable topology is a continuous map out of , 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 with the cardinality of , 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
- Separation of a topological space, connected and disconnected spaces, clopen sets, and connected subsets
- For a topological space the following agree: no separation exists, the only clopen subsets are $\varnothing$ and $X$, and every continuous map to the two-point discrete space is constant
- The discrete, indiscrete, cofinite, cocountable, particular-point and Sierpinski topologies
- Topology on a set, open and closed sets, clopen sets, the closed-set axiomatisation, and the coarser/finer comparison
- Connected components, quasicomponents, and totally disconnected spaces
- Finite, countably infinite, countable, uncountable
- $\mathbb{R}$ is uncountable (Cantor's nested intervals, 1874)
- A nonempty set is at most countable iff it is a surjective image of $\mathbb{N}$
- $\mathbb{N} \times \mathbb{N} \approx \mathbb{N}$
- Locally connected and locally path-connected spaces: a neighbourhood base of open connected, respectively open path-connected, sets at every point
- Paths, path-connected spaces and path components
- Subspace topology: the traces of the open sets, its closed sets and its bases, the continuity of the inclusion, and the characteristic property of a map into a subspace
- The components of a space are its maximal connected subsets, they partition it, and each of them is closed
- Intervals of $\mathbb{R}$: the nine order-convex forms, nondegeneracy, and length
- For a map of spaces the following agree: continuity at every point, preimages of open sets open, preimages of closed sets closed, preimages of subbasic open sets open, and $f(\overline{A}) \subseteq \overline{f(A)}$
- Continuity of a map of topological spaces at a point and globally
- Every path-connected space is connected, and every path component lies inside a component
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
- Connected space (Wikipedia) (standard reference, not scraped)
- Particular point topology (Wikipedia) (standard reference, not scraped)
- Cofinite topology (Wikipedia) (standard reference, not scraped)
- Excluded point topology (Wikipedia) (standard reference, not scraped)
- Cocountable topology (Wikipedia) (standard reference, not scraped)