## Do Short Exact Sequences of Representations Split?

We’ve seen that the category of representations is abelian, so we have all we need to talk about exact sequences. And we know that some of the most important exact sequences are short exact sequences. We also saw that every short exact sequence of vector spaces splits. So does the same hold for representations? It turns out that no, they don’t always, and I’ll give an example to show what can happen.

Consider the group of integers and the two dimensional representation defined by:

Verify for yourself that this actually does define a representation of the group of integers.

Now it’s straightforward to see that all these linear transformations send every vector of the form to itself. This defines a one-dimensional subspace fixed by the representation — a subrepresentation defined by:

Then there must be a quotient representation , and we can arrange them into a short exact sequence: . The question, then, is whether this is isomorphic to the split exact sequence . That is, can we find an isomorphism compatible with the the inclusion map from and the projection map onto ?

First off, let’s write the direct sum representation a little more explicitly. The direct sum acts on pairs of field elements by on the first and on the second, with no interaction between them. That is, we can write the representation as

And we’re looking for some isomorphism so that for every we get from the matrix to the matrix by conjugation. Explicitly, we’ll need a matrix . But we also need to make sure that as a subrepresentation of is sent to as a subrepresentation of . That is we must satisfy

Thus and right off the bat! Now the intertwining condition (equivalent to the conjugation) is that

But this says that for all , and this is clearly impossible!

So here’s an example where a short exact sequence of representations can *not* be split. At some point later we’ll see that in many cases we’re interested in they *do* split, but for now it’s good to see that they don’t always work out so nicely.

Every time I read one of your posts a tiny bit of the algebra I once was sorta-kinda proficient at comes back (ever so slightly).

Comment by jrshipley | December 17, 2008 |

That’s good! And there’s plenty more to come!

Comment by John Armstrong | December 17, 2008 |

[...] decomposable. Indeed, in categorical terms this is the statement that for some groups there are short exact sequences which do not split. To chase this down a little further, our work yesterday showed that even in the reducible case we [...]

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