The Evaluation Map

The evaluation map is a useful tool for embedding a space $X$ into a product space. In this post we demonstrate that any Tychonoff space $X$ can be embedded into a cube $I^{\mathcal{K}}$ where $I$ is the unit interval $[0,1]$ and $\mathcal{K}$ is some cardinal. Any regular space with a countable base (second-countable space) can also be embedded into the Hilbert cube $I^{\omega}$ (Urysohn’s metrization theorem). The evaluation map also plays an important role in the theory of Cech-Stone compactification.

The Evaluation Map
Let $X$ be a space. Let $\displaystyle Y=\Pi_{\alpha \in A}Y_\alpha$ be a product space. For each $y \in Y$, we use the notation $y=\langle y_\alpha \rangle_{\alpha \in A}$ to denote a point in the product space $Y$. Suppose we have a family of continuous functions $\mathcal{F}=\lbrace{f_\alpha:\alpha \in A}\rbrace$ where $f_\alpha:X \rightarrow Y_\alpha$ for each $\alpha$. Define a mapping $E_{\mathcal{F}}:X \longrightarrow \Pi_{\alpha \in A}Y_\alpha$ as follows:

For each $x \in X$, $E_{\mathcal{F}}(x)$ is the point $\langle f_\alpha(x) \rangle_{\alpha \in A} \in Y$.

This mapping is called the evaluation map of the family of continuous functions $\mathcal{F}=\lbrace{f_\alpha:\alpha \in A}\rbrace$. If the family $\mathcal{F}$ is understood, we may skip the subscript and use $E$ to denote the evaluation map.

The family of continuous functions $\mathcal{F}$ is said to separate points if for any two distinct points $x,y \in X$, there is a function $f \in \mathcal{F}$ such that $f(x) \neq f(y)$. The family of continuous functions $\mathcal{F}$ is said to separate points from closed sets if for each point $x \in X$ and for each closed set $C \subset X$ with $x \notin C$, there is a function $f \in \mathcal{F}$ such that $f(x) \notin \overline{f(C)}$.

Theorem 1. Given an evaluation map $E_{\mathcal{F}}:X \longrightarrow \Pi_{\alpha \in A}Y_\alpha$ as defined above, the following conditions hold.

1. The mapping $E_{\mathcal{F}}$ is continuous.
2. If the family of continuous functions $\mathcal{F}=\lbrace{f_\alpha:\alpha \in A}\rbrace$ separates points, then $E_{\mathcal{F}}$ is a one-to-one map.
3. If the family of continuous functions $\mathcal{F}=\lbrace{f_\alpha:\alpha \in A}\rbrace$ separates points from closed sets, then $E_{\mathcal{F}}$ is a homeomorphism from $X$ into the product space $\displaystyle Y=\Pi_{\alpha \in A}Y_\alpha$.

In this post, basic open sets in the product space $\displaystyle Y=\Pi_{\alpha \in A}Y_\alpha$ are of the form $\bigcap_{\alpha \in W} [\alpha,V_\alpha]$ where $W \subset A$ is finite, for each $\alpha \in W$, $V_\alpha$ is an open set in $Y_\alpha$ and $[\alpha,V_\alpha]=\lbrace{y \in Y:y_\alpha \in V_\alpha}\rbrace$.

Proof of 1. We show that $E_{\mathcal{F}}$ is continuous at each $x \in X$. Let $x \in X$. Let $h=\langle f_\alpha(x) \rangle_{\alpha \in A}$ and let $h \in V \cap E_{\mathcal{F}}(X)$ where $V=\bigcap_{\alpha \in W} [\alpha,V_\alpha]$ is a basic open set. Consider $U=\bigcap_{\alpha \in W} f_\alpha^{-1}(V_\alpha)$. It is easy to verify that $x \in U$ and $E_{\mathcal{F}}(U) \subset V\cap E_{\mathcal{F}}(X)$.

Proof of 2. Let $x,y \in X$ be distinct points. There is $\alpha \in A$  such that $f_\alpha(x) \neq f_\alpha(y)$. Clearly, $E_{\mathcal{F}}(x)= \langle f_\beta(x) \rangle_{\beta \in A} \neq E_{\mathcal{F}}(y)=\langle f_\beta(y) \rangle_{\beta \in A}$.

Proof of 3. Note that by condition 2 in this theorem, the map $E_{\mathcal{F}}$ is one-to-one. It suffices to show that $E_{\mathcal{F}}$ is an open map. Let $U \subset X$ be open. We show that $E_{\mathcal{F}}(U)$ is open in $E_{\mathcal{F}}(X)$. To this end, let $\langle f_\alpha(x) \rangle_{\alpha \in A} \in E_{\mathcal{F}}(U)$. Then $x \in U$. Since $\mathcal{F}$ separates points from closed sets, there is some $\beta$ such that $f_\beta(x) \notin \overline{f_\beta(X-U)}$. Let $V_\beta=Y_\beta-\overline{f_\beta(X-U)}$. Then $\langle f_\alpha(x) \rangle_{\alpha \in A} \in [\beta,V_\beta] \cap E_{\mathcal{F}}(X)=W_\beta$. We show that $W_\beta \subset E_{\mathcal{F}}(U)$. For each $\langle f_\alpha(y) \rangle_{\alpha \in A} \in W_\beta$, we have $f_\beta(y) \notin \overline{f_\beta(X-U)}$. If $y \notin U$, then $f_\beta(y) \in f_\beta(X-U)$, a contradiction. So we have $y \in U$ and this means that $\langle f_\alpha(y) \rangle_{\alpha \in A} \in E_{\mathcal{F}}(U)$. It follows that $W_\beta \subset E_{\mathcal{F}}(U)$.

Some Applications

A space $X$ is a Tychonoff space (also known as completely regular space) if for each $x \in X$ and for each closed set $C \subset X$ where $x \notin C$, there is a continuous function $f:X \rightarrow I$ such that $f(x)=1$ and $f(y)=0$ for all $y \in C$. The following is a corollary to theorem 1.

Corollary 1. Any Tychonoff space can be embedded in a cube $I^{\mathcal{K}}$.

Proof. Let $\mathcal{F}$ be the family of all continuous functions from the Tychonoff space $X$ into the unit interval $I$. By the definition of Tychonoff space, $\mathcal{F}$ separates points from closed sets. By theorem 1, the evaluation map $E_{\mathcal{F}}$ is a homeomorphism from $X$ into the cube $I^{\mathcal{K}}$ where $\mathcal{K}=\lvert \mathcal{F} \lvert$.

We now turn our attention to regular second countable space. Having a countable base has many strong properties, one of which is that it can be embedded into the Hilbert Cube $I^{\omega}=I^{\aleph_0}$. Before we prove this, observe that any regular space with a countable base is a regular Lindelof space. Furthermore, the property of having a countable base is hereditary. Thus a regular space with a countable base is hereditarily Lindelof (hence perfectly normal). The Vendenisoff Theorem states that in a perfectly normal space, every closed set is a zero-set (i.e. every open set is a cozero-set). So we make use of this theorem to obtain continuous functions that separate points from closed sets. There is a proof of The Vendenisoff Theorem in this blog. A set $Z \subset X$ is a zero-set in the space $X$ if there is a continuous function $f:X \rightarrow I$ such that $f^{-1}(0)=Z$. A set $W \subset X$ is a cozero-set if $X-W$ is a zero-set. We are now ready to prove one part of the Urysohn’s metrization theorem.

Urysohn’s metrization theorem. The following conditions are equivalent.

1. The space $X$ is a regular space with a countable base.
2. The space $X$ can be embedded into the Hilbert cube $I^{\aleph_0}$.
3. The space $X$ is a separable metric space.

We prove the direction $1 \Rightarrow 2$. Let $\lbrace{B_0,B_1,B_2,...}\rbrace$ be a countable base for the regular space $X$. Based on the preceding discussion, $X$ is perfectly normal. By the Vendenisoff Theorem, for each $n$$X-B_n$ is a zero-set. Thus for each $n$, there is a continuous function $f_n:X \rightarrow I$ such that $f_n^{-1}(0)=X-B_n$ and $f_n^{-1}((0,1])=B_n$. Let $\mathcal{F}=\lbrace{f_0,f_1,f_2,...}\rbrace$. It is easy to verify that $\mathcal{F}$ separates points from closed sets. Thus the evaluation map $E_{\mathcal{F}}$ is a homeomorphism from $X$ into $I^{\aleph_0}$.