# The Unapologetic Mathematician

## Images and Kernels

A nice quick one today. Let’s take two $G$modules $V$ and $W$. We’ll write $\hom_G(V,W)$ for the vector space of intertwinors from $V$ to $W$. This is pretty appropriate because these are the morphisms in the category of $G$-modules. It turns out that this category has kernels and has images. Those two references are pretty technical, so we’ll talk in more down-to-earth terms.

Any intertwinor $f\in\hom_G(V,W)$ is first and foremost a linear map $f:V\to W$. And as usual the kernel of $f$ is the subspace $\mathrm{Ker}(f)\subseteq V$ of vectors $v$ for which $f(v)=0$. I say that this isn’t just a subspace of $V$, but it’s a submodule as well. That is, $\mathrm{Ker}(f)$ is an invariant subspace of $V$. Indeed, we check that if $v\in\mathrm{Ker}(f)$ and $g$ is any element of $G$, then $f(gv)=gf(v)=g0=0$, so $gv\in\mathrm{Ker}(f)$ as well.

Similarly, as usual the image of $f$ is the subspace $\mathrm{Im}(f)\subseteq W$ of vectors $w$ for which there’s some $v\in V$ with $f(v)=w$. And again I say that this is an invariant subspace. Indeed, if $w=f(v)\in\mathrm{Im}(f)$ and $g$ is any element of $G$, then $gw=gf(v)=f(gv)\in\mathrm{Im}(f)$ as well.

Thus these images and kernels are not just subspaces of the vector spaces $V$ and $W$, but submodules to boot. That is, they can act as images and kernels in the category of $G$-modules just like they do in the category of complex vector spaces.

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September 29, 2010 -

## 3 Comments »

1. […] Now that we know that images and kernels of -morphisms between -modules are -modules as well, we can bring in a very general […]

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2. […] A couple days ago we mentioned the vector space . Today, we specialize to the case , where we use the usual alternate name. We write and call it the “endomorphism algebra” of . Not only is it a vector space of -morphisms, but it has a multiplication from the fact that the source and target of each one are the same and so we can compose them. […]

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3. […] of all, we can consider the kernel of a matrix representation . This is not the kernel we’ve talked about recently, which is the kernel of a -morphism. This is the kernel of a group homomorphism. In this context, […]

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