# Forgetful Functor

• Sep 8th 2011, 02:25 PM
slevvio
Forgetful Functor
Hello, I am reading some notes on category theory, and I just had a question about the forgetful functor?

Consider the forgetful functor $F: \textbf{Gp} \rightarrow \textbf{Set}$. I was wondering if the structure $\tilde{\textbf{Gp}}$ where the collection of objects is $\{ F(G) | G \in \text{Ob}(\textbf{Gp})\}$ and the collection of morphisms between two objects $F(G)$ and $F(H)$ is $\{ F(\phi) | \phi \in \text{Hom}_{\textbf{Gp}}(G,H))\}$ forms a category?

The reason I ask is because I read that $\textbf{Gp}$ was a subcategory of $\textbf{Set}$ but the objects of groups arent really contained in the objects of Sets, but I can see it if this 'image' of $\text{Gp}$ is a category. Thanks very much for any advice.
• Sep 8th 2011, 04:49 PM
NonCommAlg
Re: Forgetful Functor
Quote:

Originally Posted by slevvio
Hello, I am reading some notes on category theory, and I just had a question about the forgetful functor?

Consider the forgetful functor $F: \textbf{Gp} \rightarrow \textbf{Set}$. I was wondering if the structure $\tilde{\textbf{Gp}}$ where the collection of objects is $\{ F(G) | G \in \text{Ob}(\textbf{Gp})\}$ and the collection of morphisms between two objects $F(G)$ and $F(H)$ is $\{ F(\phi) | \phi \in \text{Hom}_{\textbf{Gp}}(G,H))\}$ forms a category?

The reason I ask is because I read that $\textbf{Gp}$ was a subcategory of $\textbf{Set}$ but the objects of groups arent really contained in the objects of Sets, but I can see it if this 'image' of $\text{Gp}$ is a category. Thanks very much for any advice.

yes it is a category because $F$ is one-to-one on the class of objects.
• Sep 8th 2011, 04:51 PM
slevvio
Re: Forgetful Functor
what about two groups (G,*) and (G,#) which are non isomorphic, but F(G,*) = F(G,#) = G? Is all we require injectivity on the class of objects?
• Sep 8th 2011, 05:00 PM
NonCommAlg
Re: Forgetful Functor
Quote:

Originally Posted by slevvio
what about two groups (G,*) and (G,#) which are non isomorphic, but F(G,*) = F(G,#) = G? Is all we require injectivity on the class of objects?

injectivity on the objects is a sufficient condition for the image of a category under a functor to be a category.
• Sep 8th 2011, 05:04 PM
slevvio
Re: Forgetful Functor
Ah ok, thanks, but that is not useful here unfortunately because the functor is not injective on the class of objects :(
• Sep 8th 2011, 05:23 PM
NonCommAlg
Re: Forgetful Functor
the only problem that occurs, if $F$ is not injective, is that we might have morphisms $\phi:G_1 \longrightarrow G_2$ and $\psi : G_3 \longrightarrow G_4$ with $F(G_2)=F(G_3)$ but $G_2 \neq G_3$. then we won't necessarily have a morphism from $G_1$ to $G_4$ but we do have a morphism $F(\phi)F(\psi) : F(G_1) \longrightarrow F(G_4).$