# How to understand the statement:”a map from the coproduct $X_1 \coprod X_2$ is equivalent to a pair of maps from $X_1$ and $X_2$”?

I’m reading up about universal properties. The following is the definition of the coproduct.

Definition. The coproduct $$X_{1} \coprod X_{2}$$ of $$X_{1}$$ and $$X_{2},$$ together with the morphisms $$i_{j}: X_{j} \rightarrow X_{1} \coprod X_{2},$$ is characterized by the following universal property: Given any object $$Y$$ with morphisms $$f_{j}: X_{j} \rightarrow Y,$$ there exists a unique $$f: X_{1} \coprod X_{2} \rightarrow Y$$ such that $$f_{j}=f \circ i_{j}$$.

The comment in the title follows this definition.

I’m not sure how to understand the term “equivalent”. I’ve been reading around and seen some relevant statements, so I guess it’s the same as $$\text{Hom}(X_1 \coprod X_2,Y) = \text{Hom}(X_1,Y) \times \text{Hom}(X_2,Y)$$? But again, my grasp on category theory is quite limited, and I’m learning these things from a group-theoretic perspective, so I’m not sure how to interpret the “$$\times$$” sign.

Say we have $$x_1 \in X_1$$ and $$x_2 \in X_2$$. How do we represent the comment above in this setup?

• The comment simply says that a map from coproduct induces a pair of maps, and vice versa: a pair of maps induces a map from coproduct. And indeed, this translates to appropriate Homs being equinumerous (just like you've written). The "$\times$" symbol is the standard Cartesian product, and it is completely valid since $Hom(X,Y)$ is a set by definition. On the other hand "$x_1\in X_1$" is meaningless since $X_1$ need not be a set (even though it is hard to imagine in maths anything that isn't a set, or set-like). – freakish Mar 22 at 10:35
• What book is this from? – Shaun Mar 22 at 14:57
• @Shaun if you’re asking about my post, then it’s just from my lecture note. – ensbana Mar 22 at 15:41
• I was. Thank you nonetheless :) – Shaun Mar 22 at 15:42

As to the last sentence: in category one doesn't talk about elements, not just objects and arrows (morphisms). In general $$X_i$$ need not even be sets for a coproduct.
There is indeed a natural bijection between $$\text{Hom}(X_1 \coprod X_2,Y) = \text{Hom}(X_1,Y) \times \text{Hom}(X_2,Y)$$: for every pair $$(f_1,f_2)$$ from the right we assign the unique $$f$$ from the diagram while to some $$g: X_1 \coprod X_2)$$ we can assign $$(g \circ i_1, g \circ i_2)$$ on the right, which is (by unicity) the inverse of the first map.
The $$\times$$ is just the standard Cartesian product in Set, so we have an isomorphism of Hom-sets in that category. So a coproduct in $$C$$ (the category we're working in) corresponds bijectively to a product in Set via Hom-sets.
• I was actually looking at this answer when thinking about the question above: math.stackexchange.com/a/2864701/120141. I’m asked to show that $\text{Ab}(G \ast H) \cong \text{Ab}(G) \times \text{Ab}(H)$, where $\text{Ab}(X)$ is the abelianization of the group $X$. The answer uses heavily universal properties using the language of Hom-sets. How do equalities of Hom-sets imply those groups are isomorphic? – ensbana Mar 22 at 12:38