## Some Review

Before we push on into our new topic, let’s look back at some of the background that we’ve already covered.

We’re talking about symmetric groups, which are, of course, groups. We have various ways of writing down an element of , including the two-line notation and the cycle notation that are covered in our earlier description of the symmetric groups. As an example, the two-line notation

and the cycle notation both describe the permutation that sends to , back to , and similarly swaps and . Similarly, the two-line notation

the composition of

and the cycle notation or (equivalently) describe the permutation that cycles the elements , , and (in that order) and leaves untouched.

We’re specifically concerned with complex representations of these groups. That is, we want to pick some complex vector space , and for each permutation we want to come up with some linear transformation for which the composition of linear transformations and the composition of permutations are “the same” in the sense that given two permutations and , the transportation corresponding to the composite is equal to the composite of the corresponding transformations .

We’re primarily interested in finite-dimensional representations. That is, ones for which is a finite-dimensional complex vector space. In this case, we know that we can always just assume that — the space of -tuples of complex numbers — and that linear transformations are described by matrices. Composition of transformations is reflected in matrix multiplication. That is, for every permutation we want to come up with an matrix so that the matrix corresponding to the composition of two permutations is the *product* of the matrices corresponding to the two permutations. I’ll be giving some more explicit examples soon.

Minor quibble: I guess the ‘n’ in $S_n$ should be (possibly) different to the dimension of the representation?

(Long-time reader: keep up the good work!)

Comment by Doormat | September 9, 2010 |

Good point. That could be confusing.

Comment by John Armstrong | September 9, 2010 |

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