---
title: Exercises in Category Theory — 1.1
published: 2016-01-29
tags: Category Theory
---

<div class="corollary">
Sets and functions do form a category $\ca{Set}$.

<div class="proof">
The objects of $\ca{Set}$ are sets and its arrows are functions.

For any set $A$ we construct $id_A$ by restricting $id$ to $A$:
$$id_A = id \upharpoonright A$$

$\circ$ is indeed associative.
</div>
</div>

<div class="corollary">
Each [poset](https://en.wikipedia.org/wiki/Partially_ordered_set) is a category.

<div class="proof">
The objects of each poset are its elements and we construct an arrow $\leq: a \to b$ for every pair of objects $(a, b)$ iff $a \leq b$.

Reflexivity ($a \leq a$) and transitivity ($a \leq b \land b \leq c \implies a \leq c$) provide a construction for $id_a$ and $\circ$, respectively.
</div>
</div>

<div class="corollary">
Each [monoid](https://en.wikipedia.org/wiki/Monoid) is a category.

<div class="proof">
The objects of each monoid are its elements and we construct for every pair of elements $(a, b)$ an arrow $\cdot b : a \to a \cdot b$.

The existence of an identity element and associativity, as required by the monoid structure, immediately provide us with a construction for $id$ and associativity of $\circ$.
</div>
</div>