# The Azimuth Project Applied Category Theory - Chapter 1 - Puzzles

## Idea

Applied Category Theory is an online course being taught by John Baez based on this free book:

Here are the puzzles for Chapter 2 of this book, often with solutions.

## Puzzles

• Puzzle 1. What is a “poset” according to Chapter 1 of Fong and Spivak’s book?

• Puzzle 2. How does their definition differ from the usual definition found in, e.g., Wikipedia or the nLab?

• Puzzle 3. What do mathematicians usually call the thing that Fong and Spivak call a poset?

• Puzzle 4. List some interesting and important examples of posets that haven’t already been listed in other comments in this thread.

• Puzzle 5. Why is this property called “trichotomy”?

• Puzzle 6. How do reflexivity and transitivity of ≤ follow from the rules of a category, if we have a category with at most one morphism from any object x to any object y, and we write x≤y when there exists a morphism from x to y?

• Puzzle 7. Why does any set with a reflexive and transitive relation ≤ yield a category with at most one morphism from any object x to any object y? That is: why are reflexivity and transitivity enough?

• Puzzle 10. There are many examples of monotone maps between posets. List a few interesting ones!

Show that if the monotone map $f: A \to B$ has an inverse $g : B \to A$ that is also a monotone map, $g$ is both a right adjoint and a left adjoint of $f$ .

• Puzzle 12. Find a right adjoint for $f$: that is, a function $g : \mathbb{N} \to \mathbb{N}$ with $f(m) \le n \text{ if and only if } m \le g(n) \text{ for all } m,n \in mathbb{N}$. How many right adjoints can you find?

• Puzzle 13. Find a left adjoint for $f$: that is

• Puzzle 14. Find the function g:ℕ→ℕ such that g(b) is the largest possible natural number a with 2a≤b.

• Puzzle 15. Find the function g:ℕ→ℕ such that g(b) is the smallest possible natural number a with b≤2a.

• Puzzle 16. What’s going on here? What’s the pattern you see, and why is it working this way?

• Puzzle 17. Show that $f_{\ast} : P X \to P Y$ is a monotone function.

• Puzzle 18. Does $f_{\ast}$ always have a left adjoint? If so, describe it. If not, give an example where it doesn’t, and some conditions under which it does have a left adjoint.

• Puzzle 19. Does $f_{\ast}$ always have a right adjoint? If so, describe it. If not, give an example where it doesn’t, and some conditions under which it does have a right adjoint.

• Puzzle 20. Does $f^{\ast}: P Y \rightarrow P X$ have a right adjoint of its own?

• Puzzle TR1. Why precisely must g(b) be a least upper bound of the set?

• Puzzle 21. Does the monotone function $i : \mathbb{N} \to \mathbb{R}$ have a left adjoint? Does it have a right adjoint? If so, what are they?

• 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.

• Puzzle 24 Show that $f_{!}: P X \rightarrow P Y$ is the right adjoint of $f^{\ast}: P X \rightarrow P Y$.

• Puzzle 28 Show that if $P$ is a partition of a set $X$, and we define a relation $\sim_P$ on $X$

• Puzzle 29 Show that if $\sim$ is an equivalence relation on a set $X$, we can define a partition $P_\sim$ on $X$

• Puzzle 31 Show that the previous two puzzles give a one-to-one correspondence between partitions of $X$ and equivalence relations on $X$.

• Puzzle 32 Proposition 1.11 of [Seven Sketches]

• Puzzle 33 Is an equivalence relation always a preorder?

category: courses