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R\mathbb{R}^{*} is homeomorphic to the unit circle by inverse stereographic projection, and N\mathbb{N}^{*} is the ordinal space ω+1\omega + 1

Example

Let X=X{}X^{*} = X \cup \{\infty\} denote the one-point compactification (The one-point (Alexandroff) compactification X=X{}X^{*} = X \cup \{\infty\}, whose open sets are the open sets of XX together with the complements in XX^{*} of the closed compact subsets of XX), whose added point is ={yX:yy}\infty = \{\, y \in X : y \notin y \,\}. Then:

  1. The naturals. Give N\mathbb{N} the discrete topology (The discrete, indiscrete, cofinite, cocountable, particular-point and Sierpinski topologies). Then N\mathbb{N}^{*} is the ordinal ω+1\omega + 1 (Ordinal addition α+β\alpha + \beta) as a set, and the topology T\mathcal{T}^{*} is the order topology of that ordinal (On an ordinal with its order topology the sets [0,β][0,\beta] and (α,β](\alpha,\beta] 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.
  2. The line. Give R\mathbb{R} its usual topology and let S1  :=  {(x,y)R2:x2+y2=1}S^1 \;:=\; \{\, (x,y) \in \mathbb{R}^2 : x^2 + y^2 = 1 \,\} carry the subspace topology from R2\mathbb{R}^2 (Rn\mathbb{R}^n as the set of functions nRn \to \mathbb{R}, and d1d_1, d2d_2, dd_\infty are metrics on it, For n1n \ge 1 the product topology on nn copies of the usual topology of R\mathbb{R} is the metric topology of dd_\infty on Rn\mathbb{R}^n, and hence also of d1d_1 and d2d_2, so Rn\mathbb{R}^n as a product and Rn\mathbb{R}^n 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 h:RS1,h(t):=(2tt2+1, t21t2+1)  (tR),h():=(0,1),h : \mathbb{R}^{*} \to S^1, \qquad h(t) := \Big(\tfrac{2t}{t^2+1},\ \tfrac{t^2-1}{t^2+1}\Big) \ \ (t \in \mathbb{R}), \qquad h(\infty) := (0,1), 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 (x,y)x/(1y)(x,y) \mapsto x/(1-y) are rational.

Facts & Assumptions

Given: N\mathbb{N} with the discrete topology, R\mathbb{R} with its usual topology, the one-point compactifications N\mathbb{N}^{*} and R\mathbb{R}^{*}, the circle S1R2S^1 \subseteq \mathbb{R}^2, and the map hh.

[L1]

T\mathcal{T}^{*} consists of the open sets of XX together with the sets XCX^{*} \setminus C for CXC \subseteq X closed in XX and a compact subset of XX; the added point is ={yX:yy}\infty = \{y \in X : y \notin y\} (The one-point (Alexandroff) compactification X=X{}X^{*} = X \cup \{\infty\}, whose open sets are the open sets of XX together with the complements in XX^{*} of the closed compact subsets of XX); and XX^{*} is compact (XX^{*} is compact and contains XX as an open subspace; XX is dense in XX^{*} exactly when XX is not compact; and XX^{*} is Hausdorff exactly when XX is locally compact and Hausdorff, claim 1).

[L2]

Every natural number satisfies nnn \notin n, N=ω\mathbb{N} = \omega is an ordinal, ω+1=ω+=ω{ω}\omega + 1 = \omega^{+} = \omega \cup \{\omega\}, and the elements of ω\omega are exactly the naturals (Basic closure properties of ordinals, Ordinal (von Neumann), ω\omega is the least limit ordinal, Ordinal addition α+β\alpha + \beta, The natural numbers N\mathbb{N} (von Neumann)).

[L5]

R2\mathbb{R}^2 carries one topology, the product topology and the metric topology of d(x,y)=max{x0y0,x1y1}d_\infty(x,y) = \max\{|x_0-y_0|, |x_1-y_1|\} being the same; a subset of R2\mathbb{R}^2 is a compact subset exactly when it is closed and bounded; and every subspace of a metrizable space is metrizable and hence Hausdorff (Rn\mathbb{R}^n as the set of functions nRn \to \mathbb{R}, and d1d_1, d2d_2, dd_\infty are metrics on it, For n1n \ge 1 the product topology on nn copies of the usual topology of R\mathbb{R} is the metric topology of dd_\infty on Rn\mathbb{R}^n, and hence also of d1d_1 and d2d_2, so Rn\mathbb{R}^n as a product and Rn\mathbb{R}^n as a metric space are one space, A subset of Rn\mathbb{R}^n 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).

[L6]

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 RR\mathbb{R} \to \mathbb{R}, 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 ARA \subseteq \mathbb{R} with the metric d(x,y)=xyd(x,y) = |x-y|, continuity and uniform continuity of f:ARf : A \to \mathbb{R} agree with the metric-space notions, the Lipschitz and Hölder conditions are the metric ones instantiated, and a subset of R\mathbb{R} is compact in the open-cover sense of R\mathbb{R} 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 f(A)f(A)f(\overline{A}) \subseteq \overline{f(A)}, Metrizable space: a topological space whose topology is induced by some metric; metrizability is topological, the metric is not, The absolute value makes R\mathbb{R} a metric space: d(x,y)=xyd(x,y) = |x-y| is a metric, its open balls are the intervals (xr,x+r)(x-r, x+r), and it is unbounded, The product set iIXi\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).

Verification

technique · direct
1.1

For X=NX = \mathbb{N} the added point is ={nN:nn}=N=ω\infty = \{\, n \in \mathbb{N} : n \notin n \,\} = \mathbb{N} = \omega by [L2], since every natural satisfies nnn \notin n. Hence N=ω{ω}=ω+=ω+1\mathbb{N}^{*} = \omega \cup \{\omega\} = \omega^{+} = \omega + 1, an equality of sets and not merely a bijection.

L1L2
1.2

For X=RX = \mathbb{R} the circle S1R2S^1 \subseteq \mathbb{R}^2 carries the subspace topology of R2\mathbb{R}^2, which is metrizable by [L5]; so S1S^1 is metrizable and hence Hausdorff by [L5]. Nothing below uses compactness of S1S^1.

L5
2.1

By [L3] the compact subsets of the discrete N\mathbb{N} are exactly its finite subsets, and every subset is closed; so by [L1] the open sets of N\mathbb{N}^{*} are the subsets of N\mathbb{N} together with the sets NF\mathbb{N}^{*} \setminus F with FNF \subseteq \mathbb{N} finite.

L1L3step 1.1
2.2

By [L4] a subset VV of ω+1\omega + 1 is open in the order topology exactly when each of its points lies in a set [0,β][0,\beta] or (α,β](\alpha,\beta] inside VV. A subset of ω\omega is open, each of its naturals nn lying in [0,0]={0}[0,0] = \{0\} or in (n1,n]={n}(n-1, n] = \{n\}; and a set VωV \ni \omega is open exactly when it contains [0,ω]=ω+1[0,\omega] = \omega+1 or some (α,ω](\alpha,\omega] with αω\alpha \in \omega, that is exactly when (ω+1)V[0,α](\omega+1) \setminus V \subseteq [0,\alpha] for some natural α\alpha, that is exactly when its complement is finite.

L2L4step 1.1
2.3

hh is a bijection RS1\mathbb{R}^{*} \to S^1. For tRt \in \mathbb{R} one computes (2t)2+(t21)2=(t2+1)2(2t)^2 + (t^2-1)^2 = (t^2+1)^2, so h(t)S1h(t) \in S^1, and h(t)(0,1)h(t) \ne (0,1) since t21=t2+1t^2 - 1 = t^2+1 is impossible. Conversely for (x,y)S1(x,y) \in S^1 with y1y \ne 1 put t:=x/(1y)t := x/(1-y); then t2=x2/(1y)2=(1y2)/(1y)2=(1+y)/(1y)t^2 = x^2/(1-y)^2 = (1-y^2)/(1-y)^2 = (1+y)/(1-y), so t2+1=2/(1y)t^2+1 = 2/(1-y) and t21=2y/(1y)t^2-1 = 2y/(1-y), whence h(t)=(x,y)h(t) = (x,y); and tt is the unique such real, being recovered from h(t)h(t) by the same formula. With h()=(0,1)h(\infty) = (0,1) this makes hh a bijection.

L8step 1.2
3.1

hh is continuous at every point of R\mathbb{R}: its two components are t2t/(t2+1)t \mapsto 2t/(t^2+1) and t(t21)/(t2+1)t \mapsto (t^2-1)/(t^2+1), quotients of polynomials whose denominator never vanishes, hence continuous by [L6], so hh restricted to R\mathbb{R} is continuous into R2\mathbb{R}^2 by [L6] and hence into the subspace S1S^1, which contains its image.

L5L6step 2.3
3.2

Claim 1 follows: by steps 2.1 and 2.2 the two topologies on the set ω+1\omega+1 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.

L1step 1.1step 2.1step 2.2
4.1

hh is continuous at \infty. Let VV be open in S1S^1 with (0,1)V(0,1) \in V; by [L5] there is a real r>0r > 0 with every point of S1S^1 at dd_\infty-distance less than rr from (0,1)(0,1) lying in VV. By [L8] fix a natural M1M \ge 1 with 1/M<r/21/M < r/2, and put C:={sR:sM}C := \{\, s \in \mathbb{R} : |s| \le M \,\}, which is a closed bounded interval of R\mathbb{R}, hence a compact subset of R\mathbb{R} by [L9]. For tCt \notin C one has t>M1|t| > M \ge 1, so 2t/(t2+1)2t/t2=2/t<2/M<r|2t/(t^2+1)| \le 2|t|/t^2 = 2/|t| < 2/M < r and (t21)/(t2+1)1=2/(t2+1)2/t22/M<r|(t^2-1)/(t^2+1) - 1| = 2/(t^2+1) \le 2/t^2 \le 2/M < r; hence h(t)Vh(t) \in V. So W:=RCW := \mathbb{R}^{*} \setminus C is open in R\mathbb{R}^{*} by [L1], contains \infty, and satisfies h[W]Vh[W] \subseteq V.

L1L5L8L9step 2.3step 3.1
5.1

hh is therefore a continuous bijection from R\mathbb{R}^{*}, which is compact by [L1], to S1S^1, 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.

L1L7step 1.2step 2.3step 3.1step 3.2step 4.1

Remarks

The naturals need no map at all. The added point of The one-point (Alexandroff) compactification X=X{}X^{*} = X \cup \{\infty\}, whose open sets are the open sets of XX together with the complements in XX^{*} of the closed compact subsets of XX is constructed from the space, and for N\mathbb{N} that construction returns ω\omega itself; the compactification is then literally the ordinal ω+1\omega+1 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, ω1\omega_1 is countably compact and sequentially compact while ω1+1\omega_1 + 1 is compact, claim 1, as it must be.

Where compactness does the work for the circle. Producing the inverse of hh 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 R\mathbb{R}^{*} and the Hausdorff property of S1S^1. 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.

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