the cedar ledge

# Clarifying Understanding of Coproducts

Date: October 2 2022

Summary: A line of questioning to ensure I understand what coproducts and disjoint unions are

Keywords: #question #zulip #category #theory #coproducts #disjoint #union #set #archive

# Bibliography

Not Available

This is a question to clarify my understand of coproducts as I have largely spent some time in understanding what they are and how to interpret them.

### Question

Hi everyone,

I was hoping I could get some clarification on my understanding of coproducts as I believe I have finally wrapped my head around them, but I want to be clear that I do. To understand coproducts, I started with the set theory definition of a coproduct:

$\bigcup_{i \in I} \{(x, i): x \in A_{i}\}$

where $i \in I$ denotes indexing from some index set, $I$, using the index term, $i$ and $A_{i}$ represents some family of sets indexed by $i$ and $x$ is some element of the respective $A_{i}$ set. Then, based on readings from Category Theory texts, I have gathered that the following is equivalent notation to the formal set theory definition: $\bigsqcup_{i \in I} A_{i}$. As an example to make sure I am on the right track, here is a small simple problem and its solution according to my understanding:

Find $\bigsqcup_{i \in I} A_{i}$ given the following:

$> I = \{1, 2, 3\} >$
$> A = \{\{1, 2, 3\}, \{2, 4, 6\}\} >$

Solution: $\bigsqcup_{i \in I} A_{i} = \{(1, 1), (2, 1), (3, 1), (2, 2), (4, 2), (6, 2)\}$

Where I am confused is the following:

1. What is the difference between $\bigsqcup_{i \in I} A_{i}$ and $\coprod_{i \in I} A_{i}$ and $X_{1} \coprod X_{2}$ syntax notation (the latter notation concerns two specific sets)?

I have seen all notations when referring to coproducts when viewed from both set theory and category theory perspectives. Is it a matter of aesthetic or is there a functional/semantic difference occurring?

1. I have read that coproducts are also known as disjoint unions.

I read about disjoint unions from a set theory perspective and also a category theory perspective but cannot really discern a functional (i.e. practical) difference from coproducts. Are coproducts and disjoint unions really effectively the same idea?

Thanks all and let me know if I can clarify anywhere!

## How To Cite

Zelko, Jacob. Clarifying Understanding of Coproducts. https://jacobzelko.com/10022022214249-understanding-coproducts. October 2 2022.

## Discussion:

CC BY-SA 4.0 Jacob Zelko. Last modified: January 17, 2023. Website built with Franklin.jl and the Julia programming language.