Lecture 9 - Adjoints and the Logic of Subsets

Lecture 9 - Adjoints and the Logic of Subsets

We've seen that classical logic is closely connected to the logic of subsets. For any set \( X \) we get a poset \( P(X) \), the power set of \(X\), whose elements are subsets of \(X\), with the partial order being \( \subseteq \). If \( X \) is a set of "states" of the world, elements of \( P(X) \) are "propositions" about the world. Less grandiosely, if \( X \) is the set of states of any system, elements of \( P(X) \) are propositions about that system.

This trick turns logical operations on propositions - like "and" and "or" - into operations on subsets, like intersection \(\cap\) and union \(\cup\). And these operations are then special cases of things we can do in other posets, too, like join \(\vee\) and meet \(\wedge\).

We could march much further in this direction. I won't, but try it yourself!

Puzzle 22. What operation on subsets corresponds to the logical operation "not"? Describe this operation in the language of posets, so it has a chance of generalizing to other posets. Based on your description, find some posets that do have a "not" operation and some that don't.

I want to march in another direction. Suppose we have a function \(f : X \to Y\) between sets. This could describe an observation, or measurement. For example, \( X \) could be the set of states of your room, and \( Y \) could be the set of states of a thermometer in your room: that is, thermometer readings. Then for any state \( x \) of your room there will be a thermometer reading, the temperature of your room, which we can call \( f(x) \).

This should yield some function between \( P(X) \), the set of propositions about your room, and \( P(Y) \), the set of propositions about your thermometer. It does. But in fact there are three such functions! And they're related in a beautiful way!

The most fundamental is this:

Definition. Suppose \(f : X \to Y \) is a function between sets. For any \( S \subseteq Y \) define its inverse image under \(f\) to be

[ f^{\ast}(S) = \{x \in X: \; f(x) \in S\} . ]

The pullback is a subset of \( X \).

The inverse image is also called the preimage, and it's often written as \(f^{-1}(S)\). That's okay, but I won't do that: I don't want to fool you into thinking \(f\) needs to have an inverse \( f^{-1} \) - it doesn't. Also, I want to match the notation in Example 1.89 of Seven Sketches.

The inverse image gives a monotone function

[ f^{\ast}: P(Y) \to P(X), ]

since if \(S,T \in P(Y)\) and \(S \subseteq T \) then

[ f^{\ast}(S) = \{x \in X: \; f(x) \in S\}
\subseteq \{x \in X:\; f(x) \in T\} = f^{\ast}(T) . ]

Why is this so fundamental? Simple: in our example, propositions about the state of your thermometer give propositions about the state of your room! If the thermometer says it's 35°, then your room is 35°, at least near your thermometer. Propositions about the measuring apparatus are useful because they give propositions about the system it's measuring - that's what measurement is all about! This explains the "backwards" nature of the function \(f^{\ast}: P(Y) \to P(X)\), going back from \(P(Y)\) to \(P(X)\).

Propositions about the system being measured also give propositions about the measurement apparatus, but this is more tricky. What does "there's a living cat in my room" tell us about the temperature I read on my thermometer? This is a bit confusing... but there is an answer because a function \(f\) really does also give a "forwards" function from \(P(X) \) to \(P(Y)\). Here it is:

Definition. Suppose \(f : X \to Y \) is a function between sets. For any \( S \subseteq X \) define its image under \(f\) to be

[ f_{!}(S) = \{y \in Y: \; y = f(x) \textrm{ for some } x \in S\} . ]

The image is a subset of \( Y \).

The image is often written as \(f(S)\), but I'm using the notation of Seven Sketches, which comes from category theory. People pronounce \(f_{!}\) as "\(f\) lower shriek".

The image gives a monotone function

[ f_{!}: P(X) \to P(Y) ]

since if \(S,T \in P(X)\) and \(S \subseteq T \) then

[ f_{!}(S) = \{y \in Y: \; y = f(x) \textrm{ for some } x \in S \}
\subseteq \{y \in Y: \; y = f(x) \textrm{ for some } x \in T \} = f_{!}(T) . ]

But here's the cool part:

Theorem. \( f_{!}: P(X) \to P(Y) \) is the left adjoint of \( f^{\ast}: P(Y) \to P(X) \).

Proof. We need to show that for any \(S \subseteq X\) and \(T \subseteq Y\) we have

[ f_{!}(S) \subseteq T \textrm{ if and only if } S \subseteq f^{\ast}(T) . ]

David Tanzer gave a quick proof in Puzzle 19. It goes like this: \(f_{!}(S) \subseteq T\) is true if and only if \(f\) maps elements of \(S\) to elements of \(T\), which is true if and only if \( S \subseteq \{x \in X: \; f(x) \in T\} = f^{\ast}(T) \). \(\quad \blacksquare\)

This is great! But there's also another way to go forwards from \(P(X)\) to \(P(Y)\), which is a right adjoint of \( f^{\ast}: P(Y) \to P(X) \). This is less widely known, and I don't even know a simple name for it. Apparently it's less useful.

Definition. Suppose \(f : X \to Y \) is a function between sets. For any \( S \subseteq X \) define

[ f_{\ast}(S) = \{y \in Y: x \in S \textrm{ for all } x \textrm{ such that } y = f(x)\} . ]

This is a subset of \(Y \).

Puzzle 23. Show that \( f_{\ast}: P(X) \to P(Y) \) is the right adjoint of \( f^{\ast}: P(Y) \to P(X) \).

What's amazing is this. Here's another way of describing our friend \(f_{!}\). For any \(S \subseteq X \) we have

[ f_{!}(S) = \{y \in Y: x \in S \textrm{ for some } x \textrm{ such that } y = f(x)\} . ]

This looks almost exactly like \(f_{\ast}\). The only difference is that while the left adjoint \(f_{!}\) is defined using "for some", the right adjoint \(f_{\ast}\) is defined using "for all". In logic "for some \(x\)" is called the existential quantifier \(\exists x\), and "for all \(x\)" is called the universal quantifier \(\forall x\). So we are seeing that existential and universal quantifiers arise as left and right adjoints!

This was discovered by Bill Lawvere in this revolutionary paper:

By now this observation is part of a big story that "explains" logic using category theory.

Two more puzzles! Let \( X \) be the set of states of your room, and \( Y \) the set of states of a thermometer in your room: that is, thermometer readings. Let \(f : X \to Y \) map any state of your room to the thermometer reading.

Puzzle 24. What is \(f_{!}(\{\text{there is a living cat in your room}\})\)? How is this an example of the "liberal" or "generous" nature of left adjoints, meaning that they're a "best approximation from above"?

Puzzle 25. What is \(f_{\ast}(\{\text{there is a living cat in your room}\})\)? How is this an example of the "conservative" or "cautious" nature of right adjoints, meaning that they're a "best approximation from below"?

To read other lectures go here.

© 2018 John Baez