Normality in Cp(X)

Any collectionwise normal space is a normal space. Any perfectly normal space is a hereditarily normal space. In general these two implications are not reversible. In function spaces C_p(X), the two implications are reversible. There is a normal space that is not countably paracompact (such a space is called a Dowker space). If a function space C_p(X) is normal, it is countably paracompact. Thus normality in C_p(X) is a strong property. This post draws on Dowker’s theorem and other results, some of them are previously discussed in this blog, to discuss this remarkable aspect of the function spaces C_p(X).

Since we are discussing function spaces, the domain space X has to have sufficient quantity of real-valued continuous functions, e.g. there should be enough continuous functions to separate the points from closed sets. The ideal setting is the class of completely regular spaces (also called Tychonoff spaces). See here for a discussion on completely regular spaces in relation to function spaces.

Let X be a completely regular space. Let C(X) be the set of all continuous functions from X into the real line \mathbb{R}. When C(X) is endowed with the pointwise convergence topology, the space is denoted by C_p(X) (see here for further comments on the definition of the pointwise convergence topology).

When Function Spaces are Normal

Let X be a completely regular space. We discuss these four facts of C_p(X):

  1. If the function space C_p(X) is normal, then C_p(X) is countably paracompact.
  2. If the function space C_p(X) is hereditarily normal, then C_p(X) is perfectly normal.
  3. If the function space C_p(X) is normal, then C_p(X) is collectionwise normal.
  4. Let X be a normal space. If C_p(X) is normal, then X has countable extent, i.e. every closed and discrete subset of X is countable, implying that X is collectionwise normal.

Fact #1 and Fact #2 rely on a representation of C_p(X) as a product space with one of the factors being the real line. For x \in X, let Y_x=\left\{f \in C_p(X): f(x)=0 \right\}. Then C_p(X) \cong Y_x \times \mathbb{R}. This representation is discussed here.

Another useful tool is Dowker’s theorem, which essentially states that for any normal space W, the space W is countably paracompact if and only if W \times C is normal for all compact metric space C if and only if W \times [0,1] is normal. For the full statement of the theorem, see Theorem 1 in this previous post, which has links to the proofs and other discussion.

To show Fact #1, suppose that C_p(X) is normal. Immediately we make use of the representation C_p(X) \cong Y_x \times \mathbb{R} where x \in X. Since Y_x \times \mathbb{R} is normal, Y_x \times [0,1] is also normal. By Dowker’s theorem, Y_x is countably paracompact. Note that Y_x is a closed subspace of the normal C_p(X). Thus Y_x is also normal.

One more helpful tool is Theorem 5 in in this previous post, which is like an extension of Dowker’s theorem, which states that a normal space W is countably paracompact if and only if W \times T is normal for any \sigma-compact metric space T. This means that Y_x \times \mathbb{R} \times \mathbb{R} is normal.

We want to show C_p(X) \cong Y_x \times \mathbb{R} is countably paracompact. Since Y_x \times \mathbb{R} \times \mathbb{R} is normal (based on the argument in the preceding paragraph), (Y_x \times \mathbb{R}) \times [0,1] is normal. Thus according to Dowker’s theorem, C_p(X) \cong Y_x \times \mathbb{R} is countably paracompact.

For Fact #2, a helpful tool is Katetov’s theorem (stated and proved here), which states that for any hereditarily normal X \times Y, one of the factors is perfectly normal or every countable subset of the other factor is closed (in that factor).

To show Fact #2, suppose that C_p(X) is hereditarily normal. With C_p(X) \cong Y_x \times \mathbb{R} and according to Katetov’s theorem, Y_x must be perfectly normal. The product of a perfectly normal space and any metric space is perfectly normal (a proof is found here). Thus C_p(X) \cong Y_x \times \mathbb{R} is perfectly normal.

The proof of Fact #3 is found in Problems 294 and 295 of [2]. The key to the proof is a theorem by Reznichenko, which states that any dense normal subspace of [0,1]^X has countable extent, hence is collectionwise normal (problem 294). See here for a proof that any normal space with countable extent is collectionwise normal (see Theorem 2). The function space C_p(X) is a dense convex subspace of [0,1]^X (problem 295). Thus if C_p(X) is normal, then it has countable extent and hence collectionwise normal.

Fact #4 says that normality of the function space imposes countable extent on the domain. This result is discussed in this previous post (see Corollary 3 and Corollary 5).

Remarks

The facts discussed here give a flavor of what function spaces are like when they are normal spaces. For further and deeper results, see [1] and [2].

Fact #1 is essentially driven by Dowker’s theorem. It follows from the theorem that whenever the product space X \times Y is normal, one of the factor must be countably paracompact if the other factor has a non-trivial convergent sequence (see Theorem 2 in this previous post). As a result, there is no Dowker space that is a C_p(X). No pathology can be found in C_p(X) with respect to finding a Dowker space. In fact, not only C_p(X) \times C is normal for any compact metric space C, it is also true that C_p(X) \times T is normal for any \sigma-compact metric space T when C_p(X) is normal.

The driving force behind Fact #2 is Katetov’s theorem, which basically says that the hereditarily normality of X \times Y is a strong statement. Coupled with the fact that C_p(X) is of the form Y_x \times \mathbb{R}, Katetov’s theorem implies that Y_x \times \mathbb{R} is perfectly normal. The argument also uses the basic fact that perfectly normality is preserved when taking product with metric spaces.

There are examples of normal but not collectionwise normal spaces (e.g. Bing’s Example G). Resolution of the question of whether normal but not collectionwise normal Moore space exists took extensive research that spanned decades in the 20th century (the normal Moore space conjecture). The function C_p(X) is outside of the scope of the normal Moore space conjecture. The function space C_p(X) is usually not a Moore space. It can be a Moore space only if the domain X is countable but then C_p(X) would be a metric space. However, it is still a powerful fact that if C_p(X) is normal, then it is collectionwise normal.

On the other hand, a more interesting point is on the normality of X. Suppose that X is a normal Moore space. If C_p(X) happens to be normal, then Fact #4 says that X would have to be collectionwise normal, which means X is metrizable. If the goal is to find a normal Moore space X that is not collectionwise normal, the normality of C_p(X) would kill the possibility of X being the example.

Reference

  1. Arkhangelskii, A. V., Topological Function Spaces, Mathematics and Its Applications Series, Kluwer Academic Publishers, Dordrecht, 1992.
  2. Tkachuk V. V., A C_p-Theory Problem Book, Topological and Function Spaces, Springer, New York, 2011.

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\copyright 2017 – Dan Ma

A useful embedding for Cp(X)

Let X be a Tychonoff space (also called completely regular space). By C_p(X) we mean the space of all continuous real-valued functions defined on X endowed with the pointwise convergence topology. In this post we discuss a scenario in which a function space can be embedded into another function space. We prove the following theorem. An example follows the proof.

Theorem 1
Suppose that the space Y is a continuous image of the space X. Then C_p(Y) can be embedded into C_p(X).

Proof of Theorem 1
Let t:X \rightarrow Y be a continuous surjection, i.e., t is a continuous function from X onto Y. Define the map \psi: C_p(Y) \rightarrow C_p(X) by \psi(f)=f \circ t for all f \in C_p(Y). We show that \psi is a homeomorphism from C_p(Y) into C_p(X).

First we show \psi is a one-to-one map. Let f,g \in C_p(Y) with f \ne g. There exists some y \in Y such that f(y) \ne g(y). Choose some x \in X such that t(x)=y. Then f \circ t \ne g \circ t since (f \circ t)(x)=f(t(x))=f(y) and (g \circ t)(x)=g(t(x))=g(y).

Next we show that \psi is continuous. Let f \in C_p(Y). Let U be open in C_p(X) with \psi(f) \in U such that

    U=\left\{q \in C_p(X): \forall \ i=1,\cdots,n, \ q(x_i) \in U_i \right\}

where x_1,\cdots,x_n are arbitrary points of X and each U_i is an open interval of the real line \mathbb{R}. Note that for each i, f(t(x_i)) \in U_i. Now consider the open set V defined by:

    V=\left\{r \in C_p(Y): \forall \ i=1,\cdots,n, \ r(t(x_i)) \in U_i \right\}

Clearly f \in V. It follows that \psi(V) \subset U since for each r \in V, it is clear that \psi(r)=r \circ t \in U.

Now we show that \psi^{-1}: \psi(C_p(Y)) \rightarrow C_p(Y) is continuous. Let \psi(f)=f \circ t \in \psi(C_p(Y)) where f \in C_p(Y). Let G be open with \psi^{-1}(f \circ t)=f \in G such that

    G=\left\{r \in C_p(Y): \forall \ i=1,\cdots,m, \ r(y_i) \in G_i \right\}

where y_1,\cdots,y_m are arbitrary points of Y and each G_i is an open interval of \mathbb{R}. Choose x_1,\cdots,x_m \in X such that t(x_i)=y_i for each i. We have f(t(x_i)) \in G_i for each i. Define the open set H by:

    H=\left\{q \in \psi(C_p(Y)) \subset C_p(X): \forall \ i=1,\cdots,m, \ q(x_i) \in G_i \right\}

Clearly f \circ t \in H. Note that \psi^{-1}(H) \subset G. To see this, let r \circ t \in H where r \in C_p(Y). Now r(t(x_i))=r(y_i) \in G_i for each i. Thus \psi^{-1}(r \circ t)=r \in G. It follows that \psi^{-1} is continuous. The proof of the theorem is now complete. \blacksquare

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Example

The proof of Theorem 1 is not difficult. It is a matter of notating carefully the open sets in both function spaces. However, the embedding makes it easy in some cases to understand certain function spaces and in some cases to relate certain function spaces.

Let \omega_1 be the first uncountable ordinal, and let \omega_1+1 be the successor ordinal to \omega_1. Furthermore consider these ordinals as topological spaces endowed with the order topology. As an application of Theorem 1, we show that C_p(\omega_1+1) can be embedded as a subspace of C_p(\omega_1). Define a continuous surjection g:\omega_1 \rightarrow \omega_1+1 as follows:

    g(\gamma) = \begin{cases} \omega_1 & \mbox{if } \ \gamma =0 \\ \gamma-1 & \mbox{if } \ 1 \le \gamma < \omega \\ \gamma & \mbox{if } \ \omega \le \gamma < \omega_1  \end{cases}

The map g is continuous from \omega_1 onto \omega_1+1. By Theorem 1, C_p(\omega_1+1) can be embedded as a subspace of C_p(\omega_1). On the other hand, C_p(\omega_1) cannot be embedded in C_p(\omega_1+1). The function space C_p(\omega_1+1) is a Frechet-Urysohn space, which is a property that is carried over to any subspace. The function C_p(\omega_1) is not Frechet-Urysohn. Thus C_p(\omega_1) cannot be embedded in C_p(\omega_1+1). A further comparison of these two function spaces is found in this subsequent post.

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\copyright \ 2014 \text{ by Dan Ma}

Cp(X) is countably tight when X is compact

Let X be a completely regular space (also called Tychonoff space). If X is a compact space, what can we say about the function space C_p(X), the space of all continuous real-valued functions with the pointwise convergence topology? When X is an uncountable space, C_p(X) is not first countable at every point. This follows from the fact that C_p(X) is a dense subspace of the product space \mathbb{R}^X and that no dense subspace of \mathbb{R}^X can be first countable when X is uncountable. However, when X is compact, C_p(X) does have a convergence property, namely C_p(X) is countably tight.

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Tightness

Let X be a completely regular space. The tightness of X, denoted by t(X), is the least infinite cardinal \kappa such that for any A \subset X and for any x \in X with x \in \overline{A}, there exists B \subset A for which \lvert B \lvert \le \kappa and x \in \overline{B}. When t(X)=\omega, we say that Y has countable tightness or is countably tight. When t(X)>\omega, we say that X has uncountable tightness or is uncountably tight. Clearly any first countable space is countably tight. There are other convergence properties in between first countability and countable tightness, e.g., the Frechet-Urysohn property. The notion of countable tightness and tightness in general is discussed in further details here.

The fact that C_p(X) is countably tight for any compact X follows from the following theorem.

Theorem 1
Let X be a completely regular space. Then the function space C_p(X) is countably tight if and only if X^n is Lindelof for each n=1,2,3,\cdots.

Theorem 1 is the countable case of Theorem I.4.1 on page 33 of [1]. We prove one direction of Theorem 1, the direction that will give us the desired result for C_p(X) where X is compact.

Proof of Theorem 1
The direction \Longleftarrow
Suppose that X^n is Lindelof for each positive integer. Let f \in C_p(X) and f \in \overline{H} where H \subset C_p(X). For each positive integer n, we define an open cover \mathcal{U}_n of X^n.

Let n be a positive integer. Let t=(x_1,\cdots,x_n) \in X^n. Since f \in \overline{H}, there is an h_t \in H such that \lvert h_t(x_j)-f(x_j) \lvert <\frac{1}{n} for all j=1,\cdots,n. Because both h_t and f are continuous, for each j=1,\cdots,n, there is an open set W(x_j) \subset X with x_j \in W(x_j) such that \lvert h_t(y)-f(y) \lvert < \frac{1}{n} for all y \in W(x_j). Let the open set U_t be defined by U_t=W(x_1) \times W(x_2) \times \cdots \times W(x_n). Let \mathcal{U}_n=\left\{U_t: t=(x_1,\cdots,x_n) \in X^n \right\}.

For each n, choose \mathcal{V}_n \subset \mathcal{U}_n be countable such that \mathcal{V}_n is a cover of X^n. Let K_n=\left\{h_t: t \in X^n \text{ such that } U_t \in \mathcal{V}_n \right\}. Let K=\bigcup_{n=1}^\infty K_n. Note that K is countable and K \subset H.

We now show that f \in \overline{K}. Choose an arbitrary positive integer n. Choose arbitrary points y_1,y_2,\cdots,y_n \in X. Consider the open set U defined by

    U=\left\{g \in C_p(X): \forall \ j=1,\cdots,n, \lvert g(y_j)-f(y_j) \lvert <\frac{1}{n} \right\}.

We wish to show that U \cap K \ne \varnothing. Choose U_t \in \mathcal{V}_n such that (y_1,\cdots,y_n) \in U_t where t=(x_1,\cdots,x_n) \in X^n. Consider the function h_t that goes with t. It is clear from the way h_t is chosen that \lvert h_t(y_j)-f(x_j) \lvert<\frac{1}{n} for all j=1,\cdots,n. Thus h_t \in K_n \cap U, leading to the conclusion that f \in \overline{K}. The proof that C_p(X) is countably tight is completed.

The direction \Longrightarrow
See Theorem I.4.1 of [1].

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Remarks

As shown above, countably tightness is one convergence property of C_p(X) that is guaranteed when X is compact. In general, it is difficult for C_p(X) to have stronger convergence properties such as the Frechet-Urysohn property. It is well known C_p(\omega_1+1) is Frechet-Urysohn. According to Theorem II.1.2 in [1], for any compact space X, C_p(X) is a Frechet-Urysohn space if and only if the compact space X is a scattered space.

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Reference

  1. Arkhangelskii, A. V., Topological Function Spaces, Mathematics and Its Applications Series, Kluwer Academic Publishers, Dordrecht, 1992.

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\copyright \ 2014 - 2015 \text{ by Dan Ma}

Working with the function space Cp(X)

This post provides basic information about the space of real-valued continuous functions with the pointwise convergence topology. The goal is to discuss the setting and to define the standard basic open sets in the function space, providing background information for subsequent posts.

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Completely Regular Spaces

The starting point is a completely regular space. A space X is said to be completely regular if X is a T_0 space and for each x \in X and for each closed subset A of X with x \notin A, there is a continuous function f:X \rightarrow [0,1] such that f(A) \subset \left\{0 \right\} and f(x)=1. Note that the T_0 axiom and the existence of the continuous function imply the T_1 axiom, which is equivalent to the property that single points are closed sets. Completely regular spaces are also called Tychonoff spaces.

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Defining the Function Space C_p(X)

Let X be a completely regular space. Let C(X) be the set of all real-valued continuous functions defined on the space X. The set C(X) is naturally a subspace of the product space \prod_{x \in X} \mathbb{R}=\mathbb{R}^X. Thus C(X) can be endowed with the subspace topology inherited from the product space \mathbb{R}^X. When this is the case, the function space C(X) is denoted by C_p(X). The topology on C_p(X) is called the pointwise convergence topology.

Now we need a good handle on the open sets in the function space C_p(X). A basic open set in the product space \mathbb{R}^X is of the form \prod_{x \in X} U_x where each U_x is an open subset of \mathbb{R} such that U_x = \mathbb{R} for all but finitely many x \in X (equivalently U_x \ne \mathbb{R} for only finitely many x \in X). Thus a basic open set in C_p(X) is of the form:

    C(X) \cap \biggl(\prod_{x \in X} U_x \biggr) \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (1)

where each U_x is an open subset of \mathbb{R} and U_x = \mathbb{R} for all but finitely many x \in X. In addition, when U_x \ne \mathbb{R}, we can take U_x to be an open interval of the form (a,b). To simplify notation, the basic open sets as described in (1) can also be notated by:

    \prod_{x \in X} U_x \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (1a)

Thus when working with open sets in C_p(X), we take \prod_{x \in X} U_x to mean the set of all f \in C(X) such that f(x) \in U_x for each x \in X.

To make the basic open sets of C_p(X) more explicit, (1) or (1a) is translated as follows:

    \bigcap_{x \in F} \ [x, O_x] \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (2)

where F \subset X is a finite set, for each x \in F, O_x is an open interval of \mathbb{R}, and [x,O_x] is the set of all f \in C(X) such that f(x) \in O_x.

There is another description of basic open sets that is useful. Let f \in C_p(X). Let F \subset X be finite. Let \epsilon>0. Let B(f,F,\epsilon) be defined as follows:

    B(f,F,\epsilon)=\left\{g \in C(X): \forall \ x \in F, \lvert f(x)-g(x) \lvert< \epsilon \right\} \ \ \ \ \ \ \ (3)

In proving results about C_p(X), we can use basic open sets that are described in any one of the three forms (1), (2) and (3). If U is a basic open subset of C_p(X), as described in (1) or (1a), we use supp(U) to denote the finite set of x \in X such that U_x \ne \mathbb{R}. The set supp(U) is called the support of U. The support for the basic open sets as described in (2) and (3) is already explicitly stated.

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Basic Discussion

The theory of C_p(X) is a vast subject area. For a systematic introduction, see [1]. One fundamental theme in function space theory is the study of how properties of X and C_p(X) are related. The domain space X and the function space C(X) are not on the same footing. The domain X only has a topological structure. The function space C_p(X) carries a topology and two natural algebraic operations of addition and multiplication, making it a topological ring. In addition, C_p(X) can be regarded as a topological group, or a linear topological space. In this post and in many subsequent posts, we narrow the focus to the topological properties of X and C_p(X), paying attention to the how the topological properties of X and C_p(X) are related.

In addition to the pointwise convergence topology, there are other topologies that can be defined on C(X), e.g., the compact-open topology, the topology of uniform convergence and others. Both the pointwise convergence topology and the compact-open topology are examples of set-open topologies. In this post and in many of the subsequent posts, the focus is on the pointwise convergence topology, i.e., the subspace topology on C(X) inherited from the product space.

The space C_p(X) automatically inherits certain properties of the product space \mathbb{R}^X. Note that C(X) is dense in \mathbb{R}^X. The product \mathbb{R}^X has the countable chain condition (CCC) since it is a product of separable spaces. Hence C_p(X) always has the CCC, i.e., there are no uncountably many pairwise disjoint open subsets of C_p(X), regardless what the domain space X is. One consequence of the CCC is that C_p(X) is paracompact if and only if C_p(X) is Lindelof.

It is well known that \mathbb{R}^X is separable if and only if the cardinality of X \le continuum. Since C(X) is dense in \mathbb{R}^X, C_p(X) is not separable if the cardinality of X > continuum. Thus C_p(X) is one way to get a CCC space that is not separable. There are non-separable C_p(X) where the cardinality of X \le continuum. Obtaining such C_p(X) would require more than the properties of the product space \mathbb{R}^X; using properties of X would be necessary.

The properties of C_p(X) discussed so far are inherited from the product space. Refer to chapter one of [1] for other elementary properties of C_p(X). See this post for a discussion of C_p(X) where X is a separable metric space. See this post about a consequence of normality of C_p(X).

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Reference

  1. Arkhangelskii, A. V., Topological Function Spaces, Mathematics and Its Applications Series, Kluwer Academic Publishers, Dordrecht, 1992.

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\copyright \ 2014 \text{ by Dan Ma}

Tietze-Urysohn-like theorems for completely regular spaces

Completely regular spaces (also called Tychonoff spaces) are topological spaces that come with a guarantee of having continuous real-valued functions in sufficient quantity. Thus the class of completely regular spaces is an ideal setting for many purposes that require the use of continuous real-valued functions (one example is working with function spaces). In a completely regular space X, for any closed set B and for any point x not in the closed set B, there always exists a continuous function f:X \rightarrow [0,1] such that f(x)=0 and f maps B to 1. It turns out that in such a space, we can replace the point x with any compact set A that is disjoint from the closed set B. This is a useful tool for proving theorems as well as for constructing objects. In this post, we discuss and prove this result (which resembles Urysohn’s lemma) and another useful fact about completely regular spaces that works very much like Tietze’s extension theorem. Specifically we prove the following results.

    Theorem 1

      Let X be a completely regular space. For any compact set A \subset X and for any closed set B \subset X that is disjoint from A, there exists a continuous function f:X \rightarrow [0,1] such that

      • f(x)=0 for all x \in A,
      • f(x)=1 for all x \in B.
    Theorem 2

      Let X be a completely regular space. For any compact set A \subset X, any continuous function f:A \rightarrow \mathbb{R} can be continuously extended over X, i.e., there exists a continuous function \hat{f}:X \rightarrow \mathbb{R} such that \hat{f}(x)=f(x) for all x \in A (in symbol we write \hat{f} \upharpoonright A=f).

A space X is normal if any two disjoint closed sets A \subset X and B \subset X can be separated by disjoint open sets, i.e., A \subset U and B \subset V for some disjoint open subsets U and V of X. Normal spaces are usually have the additional requirement that singleton sets are closed (i.e. T_1 spaces).

These two theorems remind us of two important tools for normal spaces, namely Urysohn’s lemma and Tietze’s extension theorem.

Urysohn’s lemma indicates that for any two disjoint closed sets in a normal space, the space can be mapped continuously to the closed unit interval [0,1] such that one closed set is mapped to 0 and the other closed set is mapped to 1. Theorem 1 is like a weakened version of Urysohn’s lemma in that one of the two disjoint closed sets must be compact.

Tietze’s extension theorem indicates that in a normal space, any continuous real-valued function defined on a closed subspace can be extended to the entire space. Theorem 2 is like a weakened version of Tiezte’s extension theorem in that the continuous extension only works for continuous functions defined on a compact subspace.

So if one only works in a completely regular space, one can still apply these two theorems about normal spaces (the weakened versions of course). For the sake of completeness, we state these two theorems about normal spaces.

    Urysohn’s Lemma

      Let X be a normal space. For any two disjoint closed sets A \subset X and B \subset X, there exists a continuous function f:X \rightarrow [0,1] such that

      • f(x)=0 for all x \in A,
      • f(x)=1 for all x \in B.
    Tietze’s Extension Theorem

      Let X be a normal space. For any closed set A \subset X, any continuous function f:A \rightarrow \mathbb{R} can be continuously extended over X, i.e., there exists a continuous function \hat{f}:X \rightarrow \mathbb{R} such that \hat{f}(x)=f(x) for all x \in A (in symbol we write \hat{f} \upharpoonright A=f).

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Proof of Theorem 1

We now prove Theorem 1. Let X be a completely regular space. Let A \subset X and B \subset X be two disjoint closed sets where A is compact. For each x \in A, there exists a continuous function f_x:X \rightarrow [0,1] such that f_x(x)=0 and f_x(B) \subset \left\{1 \right\}. The following collection is an open cover of the compact set A.

    \left\{f_x^{-1}([0,\frac{1}{10})): x \in A \right\}

Finitely many sets in this collection would cover A since A is compact. Choose x_1,x_2,\cdots,x_n \in A such that A \subset \bigcup \limits_{j=1}^n f_{x_n}^{-1}([0,\frac{1}{10})). Define h:X \rightarrow [0,1] by, for each x \in X, letting h(x) be the minimum of f_{x_1}(x),\cdots,f_{x_n}(x). It can be shown that the function h, being the minimum of finitely many continuous real-valued functions, is continuous. Furthermore, we have:

  • A \subset h^{-1}([0,\frac{1}{10})), and
  • h(B) \subset \left\{1 \right\}

Now define w:X \rightarrow [0,1] by, for each x \in X, letting w(x) be as follows:

    \displaystyle w(x)=\frac{10}{9} \cdot \biggl[ \text{max} \left\{h(x)-\frac{1}{10},0\right\} \biggr]

It can be shown that the function w is continuous. It is clear that w(x)=0 for all x \in A and w(x)=1 for all x \in B. \blacksquare

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Proof of Theorem 2

Interestingly, the proof of Theorem 2 given here uses Tietze’s extension theorem even Theorem 2 is described earlier as a weakened version of Tietze’s extension theorem. Beside using Tietze’s extension theorem, we also use the fact that any completely regular space can be embedded in a cube (see the previous post called Embedding Completely Regular Spaces into a Cube).

The proof is quite short once all the deep results that are used are understood. Let X be a completely regular space. Then X can be embedded in a cube, which is a product of the closed unit interval [0,1]. Thus X is homeomorphic to a subspace of the following product space

    Y=\prod \limits_{a \in S} I_a

for some index set S where I_a=[0,1] for all a \in S. We can now regard X as a subspace of the compact space Y. Let A \subset X be a compact subset of X. Let f:A \rightarrow \mathbb{R} be a continuous function.

The set A is a subset of X and can also be regarded as a subspace of the compact space Y, which is normal. Hence Tietze’s extension theorem is applicable in Y. Let \bar{f}:Y \rightarrow \mathbb{R} be a continuous extension of f. Let \hat{f}=\bar{f} \upharpoonright X. Then \hat{f} is the required continuous extension. \blacksquare

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\copyright \ 2014 \text{ by Dan Ma}

Cartesian Products of Two Paracompact Spaces

In some previous posts we discuss examples surrounding the Michael line showing that the product of a paracompact space and a complete metric space needs not be normal (see “Michael Line Basics”) and that the product of a Lindelof space and a separable metric space need not be normal (see “Bernstein Sets and the Michael Line”). These examples are classic counterexamples demonstrating that both paracompactness and Lindelofness are not preserved by taking two-factor cartesian products even when one of the factors is nice (complete metric space in the first example and separable metric space in the second example). We now show some positive results. Of course, these results require additional conditions on one or both of the factors. We prove the following results.

Result 1

    If X is paracompact and Y is compact, then X \times Y is paracompact.

Result 2

    If X is paracompact and Y is \sigma-compact, then X \times Y is paracompact.

Result 3

    If X is paracompact and perfectly normal and Y is metrizable, then X \times Y is paracompact and perfectly normal.

Result 4

    If X is hereditarily Lindelof and Y is a separable metric space, then X \times Y is hereditarily Lindelof.

With Results 1 and 2, compact spaces and \sigma-compact spaces can be called productively paracompact since the product of each of these spaces with any paracompact space is paracompact. We prove Result 1 and Result 2 below.

Result 3 and Result 4 are proved in another post Cartesian Products of Two Paracompact Spaces – Continued.

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Paracompact Spaces

First, recall some definitions. All spaces are at least regular (to us regular implies Hausdorff). Let X be a space. A collection \mathcal{A} of subsets of X is said to be a cover of X if X=\bigcup \mathcal{A} (in words every point of the space belongs to one set in the collection). Furthermore, \mathcal{A} is an open cover of X is it is a cover of X consisting of open subsets of X.

Let \mathcal{A} and \mathcal{B} be covers of the space X. The cover \mathcal{B} is said to be a refinement of \mathcal{A} (\mathcal{B} is said to refine \mathcal{A}) if for every B \in \mathcal{B}, there is some A \in \mathcal{A} such that B \subset A. The cover \mathcal{B} is said to be an open refinement of \mathcal{A} if \mathcal{B} refines \mathcal{A} and \mathcal{B} is an open cover.

A collection \mathcal{A} of subsets of X is said to be a locally finite collection if for each point x \in X, there is a non-empty open subset V of X such that x \in V and V has non-empty intersection with at most finitely many sets in \mathcal{A}. An open cover \mathcal{A} of X is said to have a locally finite open refinement if there exists an open cover \mathcal{C} of X such that \mathcal{C} refines \mathcal{A} and \mathcal{C} is a locally finite collection. We have the following definition.

Definition

    The space X is said to be paracompact if every open cover of X has a locally finite open refinement.

A collection \mathcal{U} of subsets of the space X is said to be a \sigma-locally finite collection if \mathcal{U}=\bigcup \limits_{i=1}^\infty \mathcal{U}_i such that each \mathcal{U}_i is a locally finite collection of subsets of X. Consider the property that every open cover of X has a \sigma-locally finite open refinement. This on the surface is a stronger property than paracompactness. However, Theorem 1 below shows that it is actually equivalent to paracompactness. The proof of Theorem 1 can be found in [1] (Theorem 5.1.11 in page 302) or in [2] (Theorem 20.7 in page 146).

Theorem 1
Let X be a regular space. Then X is paracompact if and only if every open cover \mathcal{U} of X has a \sigma-locally finite open refinement.

Theorem 2 below is another characterization of paracompactness that is useful. For a proof of Theorem 2, see “Finite and Countable Products of the Michael Line”.

Theorem 2
Let X be a regular space. Then X is paracompact if and only if the following holds:

    For each open cover \left\{U_t: t \in T \right\} of X, there exists a locally finite open cover \left\{V_t: t \in T \right\} such that \overline{V_t} \subset U_t for each t \in T.

Theorem 3 below shows that paracompactness is hereditary with respect to F_\sigma-subsets.

Theorem 3
Every F_\sigma-subset of a paracompact space is paracompact.

Proof of Theorem 3
Let X be paracompact. Let Y \subset X such that Y=\bigcup \limits_{i=1}^\infty Y_i where each Y_i is a closed subset of X. Let \mathcal{U} be an open cover of Y. For each U \in \mathcal{U}, let U^* be open in X such that U^* \cap Y=U.

For each i, let \mathcal{U}_i^* be the set of all U^* such that U \cap Y_i \ne \varnothing. Let \mathcal{V}_i^* be a locally finite refinement of \mathcal{U}_i^* \cup \left\{X-Y_i \right\}. Let \mathcal{V}_i be the following:

    \mathcal{V}_i=\left\{V \cap Y: V \in \mathcal{V}_i^* \text{ and } V \cap Y_i \ne \varnothing \right\}

It is clear that each \mathcal{V}_i is a locally finite collection of open set in Y covering Y_i. All the \mathcal{V}_i together form a refinement of \mathcal{U}. Thus \mathcal{V}=\bigcup \limits_{i=1}^\infty \mathcal{V}_i is a \sigma-locally finite open refinement of \mathcal{U}. By Theorem 1, the F_\sigma-set Y is paracompact. \blacksquare
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Result 1

Result 1 is the statement that:

    If X is paracompact and Y is compact, then X \times Y is paracompact.

To prove Result 1, we use the Tube lemma (for a proof, see “The Tube Lemma”).

The Tube Lemma
Let X be any space and Y be compact. For each x \in X and for each open set U \subset X \times Y such that \left\{x \right\} \times Y \subset U, there is an open set O \subset X such that \left\{x \right\} \times Y \subset O \times Y \subset U.

Proof of Result 1
Let \mathcal{U} be an open cover of X \times Y. For each x \in X, choose a finite \mathcal{U}_x \subset \mathcal{U} such that \mathcal{U}_x is a cover of \left\{x \right\} \times Y. By the Tube Lemma, for each x \in X, there is an open set O_x \subset X such that \left\{x \right\} \times Y \subset O_x \times Y \subset \cup \mathcal{U}_x. Since X is paracompact, by Theorem 2, let \left\{W_x: x \in X \right\} be a locally finite open refinement of \left\{O_x: x \in X \right\} such that W_x \subset O_x for each x \in X.

Let \mathcal{W}=\left\{(W_x \times Y) \cap U: x \in X, U \in \mathcal{U}_x \right\}. We claim that \mathcal{W} is a locally finite open refinement of \mathcal{U}. First, this is an open cover of X \times Y. To see this, let (a,b) \in X \times Y. Then a \in W_x for some x \in X. Furthermore, a \in O_x and (a,b) \in \cup \mathcal{U}_x. Thus, (a,b) \in (W_x \times Y) \cap U for some U \in \mathcal{U}_x. Secondly, it is clear that \mathcal{W} is a refinement of the original cover \mathcal{U}.

It remains to show that \mathcal{W} is locally finite. To see this, let (a,b) \in X \times Y. Then there is an open V in X such that x \in V and V can meets only finitely many W_x. Then V \times Y can meet only finitely many sets in \mathcal{W}. \blacksquare

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Result 2

Result 2 is the statement that:

    If X is paracompact and Y is \sigma-compact, then X \times Y is paracompact.

Proof of Result 2
Note that the \sigma-compact space Y is Lindelof. Since regular Lindelof are normal, Y is normal and is thus completely regular. So we can embed Y into a compact space K. For example, we can let K=\beta Y, which is the Stone-Cech compactification of Y (see “Embedding Completely Regular Spaces into a Cube”). For our purpose here, any compact space containing Y will do. By Result 1, X \times K is paracompact. Note that X \times Y can be regarded as a subspace of X \times K.

Let Y=\bigcup \limits_{i=1}^\infty Y_i where each Y_i is compact in Y. Note that X \times Y=\bigcup \limits_{i=1}^\infty X \times Y_i and each X \times Y_i is a closed subset of X \times K. Thus the product X \times Y is an F_\sigma-subset of X \times K. According to Theorem 3, F_\sigma-subsets of any paracompact space is paracompact space. Thus X \times Y is paracompact. \blacksquare

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Reference

  1. Engelking, R., General Topology, Revised and Completed edition, Heldermann Verlag, Berlin, 1989.
  2. Willard, S., General Topology, Addison-Wesley Publishing Company, 1970.

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\copyright \ \ 2012

Stone-Cech Compactifications – Another Two Characterizations

Let X be a completely regular space. Let \beta X be the Stone-Cech compactification of X. We present two characterizations of \beta X in addition to three others that are discussed previously. In all, these five characterizations can help us derive many of the basic properties of \beta X. We prove the following theorems.

Theorem C4
Let X be a completely regular space. Every two completely separated subsets of X have disjoint closures in \beta X.

Theorem U4
The property described in Theorem C4 is unique to \beta X. That is, if \alpha X is a compactification of X satisfying the condition that every two completely separated subsets of X have disjoint closures in \alpha X, then \alpha X must be \beta X.

Theorem C5
Let X be a normal space. Then every two disjoint closed subsets of X have disjoint closures in \beta X.

Theorem U5
If \alpha X is a compactification of X satisfying the property that every two disjoint closed subsets of X have disjoint closures in \alpha X, then X is normal and \alpha X must be \beta X.

The C theorem and U theorem with the same number work as a pair. The C theorem asserts that \beta X has a certain property. The corresponding U theorem asserts that of all the compactifications of X, \beta X is the only one with the property in question. Whenever we can show a given compactification does not possess the property described in the C-U theorem pair, we know that that compactification is not \beta X (consequence of the C theorem). Whenever we can show that a given compactification has the property described in the C-U theorem pair, we know that that compactification must be \beta X (a consequence of the U theorem).

Three other sets of characterizations (Theorems C1, U1, C2, U2, C3 and U3) have been established previously. See the links found below.
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Completely Separated Sets

Let Y be a completely regular space. Let H \subset Y and K \subset Y. The sets H and K are said to be completely separated in Y if there is a continuous function f:Y \rightarrow [0,1] such that for each y \in H, f(y)=0 and for each y \in K, f(y)=1 (this can also be expressed as f(H) \subset \left\{0 \right\} and f(K) \subset \left\{1 \right\}). If H and K are completely separated, \overline{H} and \overline{K} are necessarily disjoint closed sets, since \overline{H} \subset f^{-1}(0) and \overline{K} \subset f^{-1}(1).

The Urysohn’s lemma can be stated as: a space is a normal space if and only if every two disjoint closed sets are completely separated. Thus disjoint closed sets are not necessarily completely separated (such sets can be found in non-normal spaces).

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Some Helpful Results

To prove Theorem U4, we need a lemma and a theorem. Most of the work in proving Theorem U4 is carried out in Theorem 2 below.

Lemma 1
Let Y be a compact space. Let U be an open subset of Y. Let \mathcal{C} be a collection of compact subsets of Y such that \cap \mathcal{C} \subset U. Then there exists a finite collection \left\{C_1,C_2,\cdots,C_n \right\} \subset \mathcal{C} such that \bigcap \limits_{i=1}^n C_i \subset U.

Proof of Lemma 1
Let D=Y-U, which is compact. Let \mathcal{O} be the collection of all Y-C where C \in \mathcal{C}. Note that \cap \mathcal{C} \subset U implies that D \subset \cup \mathcal{O}. Thus \mathcal{O} is a collection of open sets covering the compact set D. We have \left\{O_1,O_2,\cdots,O_n \right\} \subset \mathcal{O} such that D \subset \bigcup \limits_{i=1}^n O_i. Each O_i=Y-C_i for some C_i \in \mathcal{C}. Now \left\{C_1,C_2,\cdots,C_n \right\} is the desired finite collection. \blacksquare

Theorem 2
Let T be a completely regular space. Let S be a dense subspace of T. Let f:S \rightarrow K be a continuous function from S into a compact space K. Suppose that every two completely separated subsets of S have disjoint closures in T. Then f can be extended to a continuous F:T \rightarrow K.

Proof
For each t \in T, let \mathcal{O}(t) be the set of all open subsets of T containing t. For each t \in T, let \mathcal{W}(t) be the set of all \overline{f(S \cap O)} where O \in \mathcal{O}(t). Note that each \mathcal{W}(t) consists of compact subsets of K. The theorem is established by proving the following claims.

Claim 1
For each t \in T, the collection \mathcal{W}(t) has non-empty intersection.

For any O_1, O_2, \cdots, O_n \in \mathcal{O}(t), we have the following:

    \overline{f(S \cap O_1 \cap O_2 \cap \cdots \cap O_n)} \subset \overline{f(S \cap O_1)} \cap \overline{f(S \cap O_2)} \cap \cdots \cap \overline{f(S \cap O_n)}

The above shows that \mathcal{W}(t) has the finite intersection property (f. i. p.). It is a well known fact that in a compact space, any collection of sets with f. i. p. has non-empty intersection (see [1] or [2] or see The Finite Intersection Property in Compact Spaces and Countably Compact Spaces in this blog).

Claim 2
For each t \in T, \cap \mathcal{W}(t) has only one point.

Let t \in T. Suppose that

    \left\{k_1,k_2 \right\} \subset \cap \mathcal{W}(t) where k_1 \ne k_2 \ \ \ \ \ \ \ \ \ \ \ \ \ \ (1)

Then there exist open subsets U_1 and U_2 of K such that k_1 \in U_1, k_2 \in U_2 and \overline{U_1} \cap \overline{U_2} = \varnothing. Since K is compact, it is a normal space. By the Urysohn’s lemma, there exists a continuous g:K \rightarrow [0,1] such that for each k \in \overline{U_1}, g(k)=0 and for each k \in \overline{U_2}, g(k)=1. Then because of the function g \circ f:S \rightarrow [0,1], the sets f^{-1}(\overline{U_1}) and f^{-1}(\overline{U_2}) are completely separated sets in S. By assumption, these two sets have disjoint closures in T, i.e.,

    \text{ }
    \overline{f^{-1}(\overline{U_1})} \cap \overline{f^{-1}(\overline{U_2})} = \varnothing \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (2)
    \text{ }

The point t cannot be in both of the sets in (2). Assume the following:

    \text{ }
    t \notin \overline{f^{-1}(\overline{U_1})} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (3)
    \text{ }

Then H=T- \overline{f^{-1}(\overline{U_1})} \in \mathcal{O}(t). Note that S \cap H=S-\overline{f^{-1}(\overline{U_1})}. Furthermore, \overline{f(S-\overline{f^{-1}(\overline{U_1})})} \in \mathcal{W}(t). Thus we have:

    \text{ }
    k_1 \in \cap \mathcal{W}(t) \subset \overline{f(S-\overline{f^{-1}(\overline{U_1})})}=W
    \text{ }

Since k_1 \in W and U_1 is an open set containing k_1, U_1 contains at least one point of f(S-\overline{f^{-1}(\overline{U_1})}). Choose z \in U_1 such that z \in f(S-\overline{f^{-1}(\overline{U_1})}). Now choose a \in S-\overline{f^{-1}(\overline{U_1})} such that f(a)=z. First we have a \notin \overline{f^{-1}(\overline{U_1})} and thus a \notin f^{-1}(\overline{U_1}). Secondly since f(a)=z \in U_1, we have a \in f^{-1}(U_1) \subset f^{-1}(\overline{U_1}). We now have a \notin f^{-1}(\overline{U_1}) and a \in f^{-1}(\overline{U_1}), a contradiction. If we assume t \notin \overline{f^{-1}(\overline{U_2})}, we can also derive a contradiction in a similar derivation. Thus the assumption in (1) above is faulty. The intersection \cap \mathcal{W}(t) can only have one point.

Claim 3
For each t \in S, \cap \mathcal{W}(t) =\left\{f(t) \right\}.

Let t \in S. Suppose that \cap \mathcal{W}(t) =\left\{p \right\} where p \ne f(t). the rest of the proof for Claim 3 is similar to that of Claim 2. For the sake of completeness, we give a sketch.

There exist open subsets U_1 and U_2 of K such that p \in U_1, f(t) \in U_2 and \overline{U_1} \cap \overline{U_2} = \varnothing. By the same argument as in Claim 2, we have the condition (2), i.e., \overline{f^{-1}(\overline{U_1})} \cap \overline{f^{-1}(\overline{U_2})} = \varnothing. Since t \in f^{-1}(U_2), t \notin \overline{f^{-1}(\overline{U_1})}. The remainder of the proof of Claim 3 is the same as above starting with condition (3) with p=k_1. A contradiction will be obtained. We can conclude that the assumption that \cap \mathcal{W}(t) =\left\{p \right\} where p \ne f(t) must be faulty. Thus Claim 3 is established.

Claim 4
For each t \in T, define F:T \rightarrow K by letting F(t) be the point in \cap \mathcal{W}(t). Note that this function extends f. Furthermore, the map F:T \rightarrow K is continuous.

To show F is continuous, let t \in T and let F(t) \in E where E is open in K. The collection \mathcal{W}(t) is a collection of compact subsets of K such that \left\{F(t) \right\} =\cap \mathcal{W}(t) \subset E. By Lemma 1, there exists \left\{C_1,\cdots,C_n \right\} \subset \mathcal{W}(t) such that \bigcap \limits_{i=1}^n C_i \subset E. By the definition of \mathcal{W}(t), there exists \left\{O_1,O_2,\cdots,O_n \right\} \subset \mathcal{O}(t) such that each C_i=\overline{f(S \cap O_i)}. Let O=O_1 \cap O_2 \cap \cdots \cap O_n. We have:

    \text{ }
    \overline{f(S \cap O)} \subset \bigcap \limits_{i=1}^n \overline{f(S \cap O_i)} \subset E \ \ \ \ \ \ \ \ \ \ \ \ \ \ (4)
    \text{ }

Note that O is an open subset of T and t \in O. We show that F(O) \subset E. Pick a \in O. According to the definition of \mathcal{W}(a), we have \left\{F(a) \right\}=\bigcap \limits_{U \in \mathcal{O}(a)} \overline{f(S \cap U)}. Since O \in \mathcal{O}(a), we have F(a) \in \overline{f(S \cap O)}. Thus by (4), we have F(a) \in E. Thus Claim 4 is established.

With all the above claims established, we completed the proof of Theorem 2. \blacksquare

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Theorem C4 and Theorem U4

Proof of Theorem C4
In proving C4, we use Theorem C3, which is found in C*-Embedding Property and Stone-Cech Compactification.

Let E and F be two completely separated sets in X. Then there exists some continuous g:X \rightarrow [0,1] such that for each x \in E, g(x)=0 and for each x \in F, g(x)=1. By Theorem C3, g is extended by some continuous G:\beta X \rightarrow [0,1]. The sets G^{-1}(0) and G^{-1}(1) are disjoint closed sets in \beta X. Furthermore, E \subset G^{-1}(0) and F \subset G^{-1}(1). Thus E and F have disjoint closures in \beta X. \blacksquare

Proof of Theorem U4
In proving U4, we use Theorem U1, which is stated and proved in Two Characterizations of Stone-Cech Compactification.

Suppose that \alpha X is a compactification of X satisfying the condition that every two completely separated subsets of X have disjoint closures in \alpha X. Let g:X \rightarrow Y be a continuous function from X into a compact space Y. By Theorem 2, g can be extended by a continuous G:\alpha X \rightarrow Y. By Theorem U1, \alpha X must be \beta X. \blacksquare

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Theorem C5 and Theorem U5

Proof of Theorem C5
Let X be a normal space. According to the Urysohn’s lemma, every two disjoint closed sets are completely separated. Thus by Theorem C4, every two disjoint closed subsets of X have disjoint closures in \beta X. \blacksquare

Proof of Theorem U5
Suppose that \alpha X is a compactification of X satisfying the property that every two disjoint closed subsets of X have disjoint closures in \alpha X. To show that X is normal, let H and K be disjoint closed subsets of X. By assumption about \alpha X, \overline{H} and \overline{K} (closures in \alpha X) are disjoint. Since \alpha X are compact and Hausdorff, \alpha X is normal. Then \overline{H} and \overline{K} can be separated by disjoint open subsets U and V of \alpha X. Thus U \cap X and V \cap X are disjoint open subsets of X separating H and K.

We use Theorem U4 to prove Theorem U5. We show that \alpha X satisfies Theorem U4. To this end, let E and F be two completely separated sets in X. We show that E and F have disjoint closures in \alpha X. There exists some continuous f:X \rightarrow [0,1] such that for each x \in E, f(x)=0 and for each x \in F, f(x)=1. Then f^{-1}(0) and f^{-1}(1) are disjoint closed sets in X such that E \subset f^{-1}(0) and F \subset f^{-1}(1). By assumption about \alpha X, f^{-1}(0) and f^{-1}(1) have disjoint closures in \alpha X. This implies that E and F have disjoint closures in \alpha X. Then by Theorem U4, \alpha X must be \beta X. \blacksquare

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Blog Posts on Stone-Cech Compactification

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Reference

  1. Engelking, R., General Topology, Revised and Completed edition, Heldermann Verlag, Berlin, 1989.
  2. Willard, S., General Topology, Addison-Wesley Publishing Company, 1970.

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\copyright \ \ 2012