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.
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September 8, 2010 -
Posted by John Armstrong |
Algebra, Group theory, Representation Theory, Representations of Symmetric Groups
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!)
Good point. That could be confusing.
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