Item 15 (01GR) — The Clowder Project

4.3.7 Intersections of Families of Subsets

Let $X$ be a set and let $\mathcal{U}\in \mathcal{P}(\mathcal{P}(X))$.

Definition 4.3.7.1.1 ▶ Intersections of Families of Subsets

The intersection of $\mathcal{U}$ is the set $\bigcap _{U\in \mathcal{U}}U$ defined by

\[ \bigcap _{U\in \mathcal{U}}U\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\left\{ x\in X\ \middle |\ \begin{aligned} & \text{for each $U\in \mathcal{U}$,}\\ & \text{we have $x\in U$}\end{aligned} \right\} . \]

Proposition 4.3.7.1.2 ▶ Properties of Intersections of Families of Subsets

Let $X$ be a set.

1.
Functoriality. The assignment $\mathcal{U}\mapsto \bigcap _{U\in \mathcal{U}}U$ defines a functor

\[ \bigcap \colon (\mathcal{P}(\mathcal{P}(X)),\supset )\to (\mathcal{P}(X),\subset ). \]

In particular, for each $\mathcal{U},\mathcal{V}\in \mathcal{P}(\mathcal{P}(X))$, the following condition is satisfied:
- (★)
2.
Oplax Associativity. We have a natural transformation
with components

\[ \bigcap _{A\in \mathcal{A}}\left(\bigcap _{U\in A}U\right)\subset \bigcap _{U\in \bigcap _{A\in \mathcal{A}}A}U \]

for each $\mathcal{A}\in \mathcal{P}(\mathcal{P}(\mathcal{P}(X)))$.
3.
Left Unitality. The diagram
commutes, i.e. we have

\[ \bigcap _{V\in \left\{ U\right\} }V=U. \]

for each $U\in \mathcal{P}(X)$.
4.
Oplax Right Unitality. The diagram
does not commute in general, i.e. we may have

\[ \bigcap _{\left\{ x\right\} \in \chi _{X}(U)}\left\{ x\right\} \neq U \]

in general, where $U\in \mathcal{P}(X)$. However, when $U$ is nonempty, we have

\[ \bigcap _{\left\{ x\right\} \in \chi _{X}(U)}\left\{ x\right\} \subset U. \]
5.
Interaction With Unions I. The diagram
commutes, i.e. we have

\[ \bigcap _{W\in \mathcal{U}\cup \mathcal{V}}W=\left(\bigcap _{U\in \mathcal{U}}U\right)\cap \left(\bigcap _{V\in \mathcal{V}}V\right) \]

for each $\mathcal{U},\mathcal{V}\in \mathcal{P}(\mathcal{P}(X))$.
6.
Interaction With Unions II. The diagram
commute, i.e. we have

\begin{align*} U\cup \left(\bigcap _{V\in \mathcal{V}}V\right) & = \bigcap _{V\in \mathcal{V}}(U\cup V),\\ \left(\bigcap _{U\in \mathcal{U}}U\right)\cup V & = \bigcap _{U\in \mathcal{U}}(U\cup V)\end{align*}

for each $\mathcal{U},\mathcal{V}\in \mathcal{P}(\mathcal{P}(X))$ and each $U,V\in \mathcal{P}(X)$.
7.
Interaction With Intersections I. We have a natural transformation
with components

\[ \left(\bigcap _{U\in \mathcal{U}}U\right)\cap \left(\bigcap _{V\in \mathcal{V}}V\right)\subset \bigcap _{W\in \mathcal{U}\cap \mathcal{V}}W \]

for each $\mathcal{U},\mathcal{V}\in \mathcal{P}(\mathcal{P}(X))$.
8.
Interaction With Intersections II. The diagrams
commute, i.e. we have

\begin{align*} U\cup \left(\bigcap _{V\in \mathcal{V}}V\right) & = \bigcap _{V\in \mathcal{V}}(U\cup V),\\ \left(\bigcap _{U\in \mathcal{U}}U\right)\cup V & = \bigcap _{U\in \mathcal{U}}(U\cup V)\end{align*}

for each $\mathcal{U},\mathcal{V}\in \mathcal{P}(\mathcal{P}(X))$ and each $U,V\in \mathcal{P}(X)$.
9.
Interaction With Differences. The diagram
does not commute in general, i.e. we may have

\[ \bigcap _{W\in \mathcal{U}\setminus \mathcal{V}}W\neq \left(\bigcap _{U\in \mathcal{U}}U\right)\setminus \left(\bigcap _{V\in \mathcal{V}}V\right) \]

in general, where $\mathcal{U},\mathcal{V}\in \mathcal{P}(\mathcal{P}(X))$.
10.
Interaction With Complements I. The diagram
does not commute in general, i.e. we may have

\[ \bigcap _{W\in \mathcal{U}^{\textsf{c}}}W\neq \bigcap _{U\in \mathcal{U}}U^{\textsf{c}} \]

in general, where $\mathcal{U}\in \mathcal{P}(\mathcal{P}(X))$.
11.
Interaction With Complements II. The diagram
commutes, i.e. we have

\[ \left(\bigcap _{U\in \mathcal{U}}U\right)^{\textsf{c}}=\bigcup _{U\in \mathcal{U}}U^{\textsf{c}} \]

for each $\mathcal{U}\in \mathcal{P}(\mathcal{P}(X))$.
12.
Interaction With Complements III. The diagram
commutes, i.e. we have

\[ \left(\bigcup _{U\in \mathcal{U}}U\right)^{\textsf{c}}=\bigcap _{U\in \mathcal{U}}U^{\textsf{c}} \]

for each $\mathcal{U}\in \mathcal{P}(\mathcal{P}(X))$.
13.
Interaction With Symmetric Differences. The diagram
does not commute in general, i.e. we may have

\[ \bigcap _{W\in \mathcal{U}\mathbin {\triangle }\mathcal{V}}W\neq \left(\bigcap _{U\in \mathcal{U}}U\right)\mathbin {\triangle }\left(\bigcap _{V\in \mathcal{V}}V\right) \]

in general, where $\mathcal{U},\mathcal{V}\in \mathcal{P}(\mathcal{P}(X))$.
14.
Interaction With Internal Homs I. The diagram
does not commute in general, i.e. we may have

\[ \bigcap _{W\in [\mathcal{U},\mathcal{V}]_{\mathcal{P}(X)}}W\neq \left[\bigcap _{U\in \mathcal{U}}U,\bigcap _{V\in \mathcal{V}}V\right]_{X} \]

in general, where $\mathcal{U}\in \mathcal{P}(\mathcal{P}(X))$.

15.
Interaction With Internal Homs II. The diagram
commutes, i.e. we have

\[ \left[\bigcap _{U\in \mathcal{U}}U,V\right]_{X}= \bigcup _{U\in \mathcal{U}}[U,V]_{X} \]

for each $\mathcal{U}\in \mathcal{P}(\mathcal{P}(X))$ and each $V\in \mathcal{P}(X)$.

16.

Interaction With Internal Homs III. The diagram

commutes, i.e. we have

\[ \left[U,\bigcap _{V\in \mathcal{V}}V\right]_{X}= \bigcap _{V\in \mathcal{V}}[U,V]_{X} \]

for each $U\in \mathcal{P}(X)$ and each $\mathcal{V}\in \mathcal{P}(\mathcal{P}(X))$.

17.

Interaction With Direct Images. Let $f\colon X\to Y$ be a map of sets. The diagram

commutes, i.e. we have

\[ \bigcap _{U\in \mathcal{U}}f_{!}(U)=\bigcap _{V\in f_{!}(\mathcal{U})}V \]

for each $\mathcal{U}\in \mathcal{P}(X)$, where $f_{!}(\mathcal{U})\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}(f_{!})_{!}(\mathcal{U})$.

18.

Interaction With Inverse Images. Let $f\colon X\to Y$ be a map of sets. The diagram

commutes, i.e. we have

\[ \bigcap _{V\in \mathcal{V}}f^{-1}(V)=\bigcap _{U\in f^{-1}(\mathcal{U})}U \]

for each $\mathcal{V}\in \mathcal{P}(Y)$, where $f^{-1}(\mathcal{V})\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}(f^{-1})^{-1}(\mathcal{V})$.

19.

Interaction With Codirect Images. Let $f\colon X\to Y$ be a map of sets. The diagram

commutes, i.e. we have

\[ \bigcap _{U\in \mathcal{U}}f_{*}(U)=\bigcap _{V\in f_{*}(\mathcal{U})}V \]

for each $\mathcal{U}\in \mathcal{P}(X)$, where $f_{*}(\mathcal{U})\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}(f_{*})_{*}(\mathcal{U})$.

20.

Interaction With Unions of Families I. The diagram

commutes, i.e. we have

\[ \bigcap _{U\in {\scriptsize \displaystyle \bigcup _{A\in \mathcal{A}}A}}U=\bigcap _{A\in \mathcal{A}}\left(\bigcap _{U\in A}U\right) \]

for each $\mathcal{A}\in \mathcal{P}(\mathcal{P}(X))$.

21.

Interaction With Unions of Families II. Let $X$ be a set and consider the compositions

given by

\[ \begin{aligned} \mathcal{A} & \mapsto \bigcup _{U\in {\scriptsize \displaystyle \bigcap _{A\in \mathcal{A}}}A}U,\\ \mathcal{A} & \mapsto \bigcup _{A\in \mathcal{A}}\left(\bigcap _{U\in A}U\right), \end{aligned} \quad \begin{aligned} \mathcal{A} & \mapsto \bigcap _{U\in {\scriptsize \displaystyle \bigcup _{A\in \mathcal{A}}A}}U,\\ \mathcal{A} & \mapsto \bigcap _{A\in \mathcal{A}}\left(\bigcup _{U\in A}U\right) \end{aligned} \]

for each $\mathcal{A}\in \mathcal{P}(\mathcal{P}(\mathcal{P}(X)))$. We have the following inclusions:

All other possible inclusions fail to hold in general.

Proof of Proposition 4.3.6.1.2.

Item 1: Functoriality

Since $\mathcal{P}(X)$ is posetal, it suffices to prove the condition $(\star )$. So let $\mathcal{U},\mathcal{V}\in \mathcal{P}(\mathcal{P}(X))$ with $\mathcal{U}\subset \mathcal{V}$. We claim that

\[ \bigcap _{V\in \mathcal{V}}V\subset \bigcap _{U\in \mathcal{U}}U. \]

Indeed, if $x\in \bigcap _{V\in \mathcal{V}}V$, then $x\in V$ for all $V\in \mathcal{V}$. But since $\mathcal{U}\subset \mathcal{V}$, it follows that $x\in U$ for all $U\in \mathcal{U}$ as well. Thus $x\in \bigcap _{U\in \mathcal{U}}U$, which gives our desired inclusion.

Item 2: Oplax Associativity

We have

\begin{align*} \bigcap _{A\in \mathcal{A}}\left(\bigcap _{U\in A}U\right) & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\left\{ x\in X\ \middle |\ \begin{aligned} & \text{for each $A\in \mathcal{A}$,}\\ & \text{we have $x\in \bigcap _{U\in A}U$} \end{aligned} \right\} \\ & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\left\{ x\in X\ \middle |\ \begin{aligned} & \text{for each $A\in \mathcal{A}$ and each}\\ & \text{$U\in A$, we have $x\in U$} \end{aligned} \right\} \\ & = \left\{ x\in X\ \middle |\ \begin{aligned} & \text{for each $U\in \displaystyle \bigcup _{A\in \mathcal{A}}A$,}\\ & \text{we have $x\in U$} \end{aligned} \right\} \\ & \subset \left\{ x\in X\ \middle |\ \begin{aligned} & \text{for each $U\in \displaystyle \bigcap _{A\in \mathcal{A}}A$,}\\ & \text{we have $x\in U$} \end{aligned} \right\} \\ & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\bigcap _{U\in {\scriptsize \displaystyle \bigcap _{A\in \mathcal{A}}A}}U. \end{align*}

Since $\mathcal{P}(X)$ is posetal, naturality is automatic (Chapter 11: Categories, Item 4 of Proposition 11.2.7.1.2). This finishes the proof.

Item 3: Left Unitality

We have

\begin{align*} \bigcap _{V\in \left\{ U\right\} }V & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\left\{ x\in X\ \middle |\ \begin{aligned} & \text{for each $V\in \left\{ U\right\} $,}\\ & \text{we have $x\in U$} \end{aligned} \right\} \\ & = \left\{ x\in X\ \middle |\ x\in U \right\} \\ & = U.\end{align*}

This finishes the proof.

Item 4: Oplax Right Unitality

If $U=\text{Ø}$, then we have

\begin{align*} \bigcap _{\left\{ u\right\} \in \chi _{X}(U)}\left\{ u\right\} & = \bigcap _{\left\{ u\right\} \in \text{Ø}}\left\{ u\right\} \\ & = X,\end{align*}

so $\bigcap _{\left\{ u\right\} \in \chi _{X}(U)}\left\{ u\right\} =X\neq \text{Ø}=U$. When $U$ is nonempty, we have two cases:

1.
If $U$ is a singleton, say $U=\left\{ u\right\} $, we have

\begin{align*} \bigcap _{\left\{ u\right\} \in \chi _{X}(U)}\left\{ u\right\} & = \left\{ u\right\} \\ & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}U.\end{align*}
2.
If $U$ contains at least two elements, we have

\begin{align*} \bigcap _{\left\{ u\right\} \in \chi _{X}(U)}\left\{ u\right\} & = \text{Ø}\\ & \subset U.\end{align*}

This finishes the proof.

Item 5: Interaction With Unions I

We have

\begin{align*} \bigcap _{W\in \mathcal{U}\cup \mathcal{V}}W & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\left\{ x\in X\ \middle |\ \begin{aligned} & \text{for each $W\in \mathcal{U}\cup \mathcal{V}$,}\\ & \text{we have $x\in W$} \end{aligned} \right\} \\ & = \left\{ x\in X\ \middle |\ \begin{aligned} & \text{for each $W\in \mathcal{U}$ and each}\\ & \text{$W\in \mathcal{V}$, we have $x\in W$} \end{aligned} \right\} \\ & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\left\{ x\in X\ \middle |\ \begin{aligned} & \text{for each $W\in \mathcal{U}$,}\\ & \text{we have $x\in W$} \end{aligned} \right\} \\ & \phantom{={}}\mkern 4mu\cap \left\{ x\in X\ \middle |\ \begin{aligned} & \text{for each $W\in \mathcal{V}$,}\\ & \text{we have $x\in W$} \end{aligned} \right\} \\ & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\left(\bigcap _{W\in \mathcal{U}}W\right)\cap \left(\bigcap _{W\in \mathcal{V}}W\right)\\ & = \left(\bigcap _{U\in \mathcal{U}}U\right)\cap \left(\bigcap _{V\in \mathcal{V}}V\right). \end{align*}

This finishes the proof.

Item 6: Interaction With Unions II

Omitted.

Item 7: Interaction With Intersections I

We have

\begin{align*} \left(\bigcap _{U\in \mathcal{U}}U\right)\cap \left(\bigcap _{V\in \mathcal{V}}V\right) & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\left\{ x\in X\ \middle |\ \begin{aligned} & \text{for each $U\in \mathcal{U}$,}\\ & \text{we have $x\in U$} \end{aligned} \right\} \\ & \phantom{={}}\mkern 4mu\cup \left\{ x\in X\ \middle |\ \begin{aligned} & \text{for each $V\in \mathcal{V}$,}\\ & \text{we have $x\in V$} \end{aligned} \right\} \\ & = \left\{ x\in X\ \middle |\ \begin{aligned} & \text{for each $W\in \mathcal{U}\cap \mathcal{V}$,}\\ & \text{we have $x\in W$} \end{aligned} \right\} \\ & \subset \left\{ x\in X\ \middle |\ \begin{aligned} & \text{for each $W\in \mathcal{U}\cup \mathcal{V}$,}\\ & \text{we have $x\in W$} \end{aligned} \right\} \\ & \mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}\bigcap _{W\in \mathcal{U}\cap \mathcal{V}}W.\end{align*}

Since $\mathcal{P}(X)$ is posetal, naturality is automatic (Chapter 11: Categories, Item 4 of Proposition 11.2.7.1.2). This finishes the proof.

Item 8: Interaction With Intersections II

Omitted.

Item 9: Interaction With Differences

Let $X=\left\{ 0,1\right\} $, let $\mathcal{U}=\left\{ \left\{ 0\right\} ,\left\{ 0,1\right\} \right\} $, and let $\mathcal{V}=\left\{ \left\{ 0\right\} \right\} $. We have

\begin{align*} \bigcap _{W\in \mathcal{U}\setminus \mathcal{V}}U & = \bigcap _{W\in \left\{ \left\{ 0,1\right\} \right\} }W\\ & = \left\{ 0,1\right\} , \end{align*}

whereas

\begin{align*} \left(\bigcap _{U\in \mathcal{U}}U\right)\setminus \left(\bigcap _{V\in \mathcal{V}}V\right) & = \left\{ 0\right\} \setminus \left\{ 0\right\} \\ & = \text{Ø}. \end{align*}

Thus we have

\[ \bigcap _{W\in \mathcal{U}\setminus \mathcal{V}}W=\left\{ 0,1\right\} \neq \text{Ø}=\left(\bigcap _{U\in \mathcal{U}}U\right)\setminus \left(\bigcap _{V\in \mathcal{V}}V\right). \]

This finishes the proof.

Item 10: Interaction With Complements I

Let $X=\left\{ 0,1\right\} $ and let $\mathcal{U}=\left\{ \left\{ 0\right\} \right\} $. We have

\begin{align*} \bigcap _{W\in \mathcal{U}^{\textsf{c}}}U & = \bigcap _{W\in \left\{ \text{Ø},\left\{ 1\right\} ,\left\{ 0,1\right\} \right\} }W\\ & = \text{Ø}, \end{align*}

whereas

\begin{align*} \bigcap _{U\in \mathcal{U}}U^{\textsf{c}} & = \left\{ 0\right\} ^{\textsf{c}}\\ & = \left\{ 1\right\} . \end{align*}

Thus we have

\[ \bigcap _{W\in \mathcal{U}^{\textsf{c}}}U=\text{Ø}\neq \left\{ 1\right\} =\bigcap _{U\in \mathcal{U}}U^{\textsf{c}}. \]

This finishes the proof.

Item 11: Interaction With Complements II

This is a repetition of Item 12 of Proposition 4.3.6.1.2 and is proved there.

Item 12: Interaction With Complements III

This is a repetition of Item 11 of Proposition 4.3.6.1.2 and is proved there.

Item 13: Interaction With Symmetric Differences

Let $X=\left\{ 0,1\right\} $, let $\mathcal{U}=\left\{ \left\{ 0,1\right\} \right\} $, and let $\mathcal{V}=\left\{ \left\{ 0\right\} ,\left\{ 0,1\right\} \right\} $. We have

\begin{align*} \bigcap _{W\in \mathcal{U}\mathbin {\triangle }\mathcal{V}}W & = \bigcap _{W\in \left\{ \left\{ 0\right\} \right\} }W\\ & = \left\{ 0\right\} , \end{align*}

whereas

\begin{align*} \left(\bigcap _{U\in \mathcal{U}}U\right)\mathbin {\triangle }\left(\bigcap _{V\in \mathcal{V}}V\right) & = \left\{ 0,1\right\} \mathbin {\triangle }\left\{ 0\right\} \\ & = \text{Ø}, \end{align*}

Thus we have

\[ \bigcap _{W\in \mathcal{U}\mathbin {\triangle }\mathcal{V}}W=\left\{ 0\right\} \neq \text{Ø}=\left(\bigcap _{U\in \mathcal{U}}U\right)\mathbin {\triangle }\left(\bigcap _{V\in \mathcal{V}}V\right). \]

This finishes the proof.

Item 14: Interaction With Internal Homs I

This is a repetition of Item 10 of Proposition 4.4.7.1.3 and is proved there.

Item 15: Interaction With Internal Homs II

This is a repetition of Item 11 of Proposition 4.4.7.1.3 and is proved there.

Item 16: Interaction With Internal Homs III

This is a repetition of Item 12 of Proposition 4.4.7.1.3 and is proved there.

Item 17: Interaction With Direct Images

This is a repetition of Item 4 of Proposition 4.6.1.1.5 and is proved there.

Item 18: Interaction With Inverse Images

This is a repetition of Item 4 of Proposition 4.6.2.1.3 and is proved there.

Item 19: Interaction With Codirect Images