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.
is homeomorphic to the unit circle by inverse stereographic projection, and is the ordinal space
Example
Let denote the one-point compactification (The one-point (Alexandroff) compactification , whose open sets are the open sets of together with the complements in of the closed compact subsets of ), whose added point is . Then:
- The naturals. Give the discrete topology (The discrete, indiscrete, cofinite, cocountable, particular-point and Sierpinski topologies). Then is the ordinal (Ordinal addition ) as a set, and the topology is the order topology of that ordinal (On an ordinal with its order topology the sets and form a basis of clopen sets, the isolated points are exactly the non-limit ordinals, and the space is Hausdorff); so the identity map is a homeomorphism (Homeomorphism, open map, closed map, embedding, and what it means for a property to be topological) and no construction is needed.
- The line. Give its usual topology and let carry the subspace topology from ( as the set of functions , and , , are metrics on it, For the product topology on copies of the usual topology of is the metric topology of on , and hence also of and , so as a product and as a metric space are one space, 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 map the inverse of stereographic projection from the north pole, is a homeomorphism.
No trigonometry is used, and no circle is described by angles; the map above and its inverse are rational.
Facts & Assumptions
Given: with the discrete topology, with its usual topology, the one-point compactifications and , the circle , and the map .
consists of the open sets of together with the sets for closed in and a compact subset of ; the added point is (The one-point (Alexandroff) compactification , whose open sets are the open sets of together with the complements in of the closed compact subsets of ); and is compact ( is compact and contains as an open subspace; is dense in exactly when is not compact; and is Hausdorff exactly when is locally compact and Hausdorff, claim 1).
Every natural number satisfies , is an ordinal, , and the elements of are exactly the naturals (Basic closure properties of ordinals, Ordinal (von Neumann), is the least limit ordinal, Ordinal addition , The natural numbers (von Neumann)).
In the discrete topology every subset is open and closed, every subspace is discrete, and a discrete space is compact exactly when it is finite (The discrete, indiscrete, cofinite, cocountable, particular-point and Sierpinski topologies, 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 discrete, indiscrete, cofinite, cocountable, particular-point and Sierpinski topologies placed in the compactness hierarchy, claim 1; Open cover, subcover, and compact topological space; a compact subset is a subspace that is compact in its own right).
On an ordinal the sets and form a basis for the order topology, so a subset of is open exactly when each of its points lies in one of them inside it (On an ordinal with its order topology the sets and form a basis of clopen sets, the isolated points are exactly the non-limit ordinals, and the space is Hausdorff, claim 1; Every successor ordinal is compact in its order topology and every limit ordinal is not; and, assuming countable choice, is countably compact and sequentially compact while is compact).
carries one topology, the product topology and the metric topology of being the same; a subset of is a compact subset exactly when it is closed and bounded; and every subspace of a metrizable space is metrizable and hence Hausdorff ( as the set of functions , and , , are metrics on it, For the product topology on copies of the usual topology of is the metric topology of on , and hence also of and , so as a product and as a metric space are one space, A subset of with the product topology is compact exactly when it is closed and bounded, the product topology being the Euclidean metric topology, Metrizable space: a topological space whose topology is induced by some metric; metrizability is topological, the metric is not, Every subspace of a metrizable space is metrizable and every subspace of a first countable space is first countable, the metric case being the subspace metric already identified with the subspace topology, Hausdorff space: distinct points have disjoint open neighbourhoods; every metrizable space is Hausdorff and the indiscrete topology on two points is not, 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, Open ball, closed ball and sphere in a metric space, Bounded subset, diameter, distance from a point to a set, and distance between two sets in a metric space).
A map into a product is continuous exactly when both components are; a quotient of polynomial functions with nowhere vanishing denominator is continuous as a map , and continuity there agrees with continuity as a map of metric spaces and hence of topological spaces (A map into a product is continuous iff each of its components is; the projections are continuous and open; and each projection is surjective when every factor is nonempty, which for an infinite index set uses the Axiom of Choice, claim 2; Sums, scalar multiples, products, absolute values, maxima, minima and quotients with nonvanishing denominator of continuous functions are continuous, as are constants, the identity and every polynomial function, Dictionary: for with the metric , continuity and uniform continuity of agree with the metric-space notions, the Lipschitz and Hölder conditions are the metric ones instantiated, and a subset of is compact in the open-cover sense of exactly when it is a compact metric subspace, claim 1; Continuity of a map of topological spaces at a point and globally, 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 , Metrizable space: a topological space whose topology is induced by some metric; metrizability is topological, the metric is not, The absolute value makes a metric space: is a metric, its open balls are the intervals , and it is unbounded, The product set of functions choosing a point in each factor, the projections, the box topology, and the product topology as the initial topology of the projections; the empty product is a one-point space).
A continuous bijection from a compact space to a Hausdorff space is a homeomorphism (A continuous image of a compact space is compact; a continuous real-valued map on a nonempty compact space attains a maximum and a minimum; and a continuous bijection from a compact space to a Hausdorff space is a homeomorphism, claim 3; Homeomorphism, open map, closed map, embedding, and what it means for a property to be topological).
For every real there is a natural with , and is an ordered field (For every in a complete ordered field there is a natural with , The canonical natural of a field, Complete ordered field (least-upper-bound property), Intervals of : the nine order-convex forms, nondegeneracy, and length, Topology on a set, open and closed sets, clopen sets, the closed-set axiomatisation, and the coarser/finer comparison).
A closed bounded interval of is compact in the open-cover sense of real analysis (Heine-Borel by bisection: every closed bounded interval is compact); that notion agrees with metric compactness (claim 5 of Dictionary: for with the metric , continuity and uniform continuity of agree with the metric-space notions, the Lipschitz and Hölder conditions are the metric ones instantiated, and a subset of is compact in the open-cover sense of exactly when it is a compact metric subspace), which in turn agrees with compactness of the subspace in the topological sense (claim 2 of For a metric space with its metric topology, compactness in the topological sense is compactness in the metric sense, and the two notions of compact subset coincide), so such an interval is a compact subset of the topological space .
Verification
For the added point is by [L2], since every natural satisfies . Hence , an equality of sets and not merely a bijection.
For the circle carries the subspace topology of , which is metrizable by [L5]; so is metrizable and hence Hausdorff by [L5]. Nothing below uses compactness of .
By [L3] the compact subsets of the discrete are exactly its finite subsets, and every subset is closed; so by [L1] the open sets of are the subsets of together with the sets with finite.
By [L4] a subset of is open in the order topology exactly when each of its points lies in a set or inside . A subset of is open, each of its naturals lying in or in ; and a set is open exactly when it contains or some with , that is exactly when for some natural , that is exactly when its complement is finite.
is a bijection . For one computes , so , and since is impossible. Conversely for with put ; then , so and , whence ; and is the unique such real, being recovered from by the same formula. With this makes a bijection.
is continuous at every point of : its two components are and , quotients of polynomials whose denominator never vanishes, hence continuous by [L6], so restricted to is continuous into by [L6] and hence into the subspace , which contains its image.
Claim 1 follows: by steps 2.1 and 2.2 the two topologies on the set of step 1.1 are the same family of subsets, so the identity map is a bijection carrying open sets to open sets in both directions and is a homeomorphism.
is continuous at . Let be open in with ; by [L5] there is a real with every point of at -distance less than from lying in . By [L8] fix a natural with , and put , which is a closed bounded interval of , hence a compact subset of by [L9]. For one has , so and ; hence . So is open in by [L1], contains , and satisfies .
is therefore a continuous bijection from , which is compact by [L1], to , which is Hausdorff by step 1.2; so [L7] makes it a homeomorphism, which is claim 2. With claim 1 at step 3.2 both statements are proved.
Remarks
The naturals need no map at all. The added point of The one-point (Alexandroff) compactification , whose open sets are the open sets of together with the complements in of the closed compact subsets of is constructed from the space, and for that construction returns itself; the compactification is then literally the ordinal with its order topology, which is compact by Every successor ordinal is compact in its order topology and every limit ordinal is not; and, assuming countable choice, is countably compact and sequentially compact while is compact, claim 1, as it must be.
Where compactness does the work for the circle. Producing the inverse of explicitly is possible here and was done at step 2.3, but continuity of that inverse is never checked: [L7] supplies it from compactness of and the Hausdorff property of . That is the standard use of claim 3 of A continuous image of a compact space is compact; a continuous real-valued map on a nonempty compact space attains a maximum and a minimum; and a continuous bijection from a compact space to a Hausdorff space is a homeomorphism.
Depends on
- Heine-Borel by bisection: every closed bounded interval $[a,b]$ is compact
- The one-point (Alexandroff) compactification $X^{*} = X \cup \{\infty\}$, whose open sets are the open sets of $X$ together with the complements in $X^{*}$ of the closed compact subsets of $X$
- $X^{*}$ is compact and contains $X$ as an open subspace; $X$ is dense in $X^{*}$ exactly when $X$ is not compact; and $X^{*}$ is Hausdorff exactly when $X$ is locally compact and Hausdorff
- On an ordinal with its order topology the sets $[0,\beta]$ and $(\alpha,\beta]$ form a basis of clopen sets, the isolated points are exactly the non-limit ordinals, and the space is Hausdorff
- Every successor ordinal is compact in its order topology and every limit ordinal is not; and, assuming countable choice, $\omega_1$ is countably compact and sequentially compact while $\omega_1 + 1$ is compact
- Ordinal addition $\alpha + \beta$
- $\omega$ is the least limit ordinal
- Basic closure properties of ordinals
- Ordinal (von Neumann)
- The discrete, indiscrete, cofinite, cocountable, particular-point and Sierpinski topologies
- The natural numbers $\mathbb{N}$ (von Neumann)
- Open cover, subcover, and compact topological space; a compact subset is a subspace that is compact in its own right
- 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
- Homeomorphism, open map, closed map, embedding, and what it means for a property to be topological
- A continuous image of a compact space is compact; a continuous real-valued map on a nonempty compact space attains a maximum and a minimum; and a continuous bijection from a compact space to a Hausdorff space is a homeomorphism
- Hausdorff space: distinct points have disjoint open neighbourhoods; every metrizable space is Hausdorff and the indiscrete topology on two points is not
- Metrizable space: a topological space whose topology is induced by some metric; metrizability is topological, the metric is not
- Every subspace of a metrizable space is metrizable and every subspace of a first countable space is first countable, the metric case being the subspace metric already identified with the subspace topology
- The absolute value makes $\mathbb{R}$ a metric space: $d(x,y) = |x-y|$ is a metric, its open balls are the intervals $(x-r, x+r)$, and it is unbounded
- 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
- Open ball, closed ball and sphere in a metric space
- $\mathbb{R}^n$ as the set of functions $n \to \mathbb{R}$, and $d_1$, $d_2$, $d_\infty$ are metrics on it
- For $n \ge 1$ the product topology on $n$ copies of the usual topology of $\mathbb{R}$ is the metric topology of $d_\infty$ on $\mathbb{R}^n$, and hence also of $d_1$ and $d_2$, so $\mathbb{R}^n$ as a product and $\mathbb{R}^n$ as a metric space are one space
- A subset of $\mathbb{R}^n$ with the product topology is compact exactly when it is closed and bounded, the product topology being the Euclidean metric topology
- Continuity of a map of topological spaces at a point and globally
- 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 $f(\overline{A}) \subseteq \overline{f(A)}$
- A map into a product is continuous iff each of its components is; the projections are continuous and open; and each projection is surjective when every factor is nonempty, which for an infinite index set uses the Axiom of Choice
- Sums, scalar multiples, products, absolute values, maxima, minima and quotients with nonvanishing denominator of continuous functions are continuous, as are constants, the identity and every polynomial function
- Dictionary: for $A \subseteq \mathbb{R}$ with the metric $d(x,y) = |x-y|$, continuity and uniform continuity of $f : A \to \mathbb{R}$ agree with the metric-space notions, the Lipschitz and Hölder conditions are the metric ones instantiated, and a subset of $\mathbb{R}$ is compact in the open-cover sense of $\mathbb{R}$ exactly when it is a compact metric subspace
- For every $\varepsilon > 0$ in a complete ordered field there is a natural $n \ge 1$ with $1/n < \varepsilon$
- The canonical natural $\iota(n) = n \cdot 1_F$ of a field
- Topology on a set, open and closed sets, clopen sets, the closed-set axiomatisation, and the coarser/finer comparison
- Intervals of $\mathbb{R}$: the nine order-convex forms, nondegeneracy, and length
- The product set $\prod_{i \in I} X_i$ of functions choosing a point in each factor, the projections, the box topology, and the product topology as the initial topology of the projections; the empty product is a one-point space
- Bounded subset, diameter, distance from a point to a set, and distance between two sets in a metric space
- Complete ordered field (least-upper-bound property)
- The discrete, indiscrete, cofinite, cocountable, particular-point and Sierpinski topologies placed in the compactness hierarchy
- For a metric space with its metric topology, compactness in the topological sense is compactness in the metric sense, and the two notions of compact subset coincide
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: 241 results over 31 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
- Alexandroff extension (Wikipedia) (standard reference, not scraped)
- Stereographic projection (Wikipedia) (standard reference, not scraped)
- I. Khatchatourian, Compactifications (MAT327 notes) (standard reference, not scraped)