character table of
Let’s use GAP
to find the character table of
gap> G:=SymmetricGroup(5);
Sym( [ 1 .. 5 ] )
gap> tab:=CharacterTable(G);
CharacterTable( Sym( [ 1 .. 5 ] ) )
gap> Display(tab);
CT1
2 3 2 3 1 1 2 .
3 1 1 . 1 1 . .
5 1 . . . . . 1
1a 2a 2b 3a 6a 4a 5a
2P 1a 1a 1a 3a 3a 2b 5a
3P 1a 2a 2b 1a 2a 4a 5a
5P 1a 2a 2b 3a 6a 4a 1a
X.1 1 -1 1 1 -1 -1 1
X.2 4 -2 . 1 1 . -1
X.3 5 -1 1 -1 -1 1 .
X.4 6 . -2 . . . 1
X.5 5 1 1 -1 1 -1 .
X.6 4 2 . 1 -1 . -1
X.7 1 1 1 1 1 1 1
gap>
Diaconis example – survey data
This data is taken from the paper (Diaconis 1989)
It describes 5,738 completed ballots rank-ordering 5 candidates.
View a rank-ordered ballot as an element of the symmetric group
first ranking table
the regular representation
This diagram shows the decomposition of the regular representation into isotypic components.
Be careful: the notation Diaconis is using here does not match that
used by GAP
above. For example, the representation Diaconis writes
as X.5
by GAP
.
The second row reflects the decomposition of the frequency function
The second row entries are the “sums of squares”
Remember that we can compute the
For example,
More generally, if
Note that
normalizing the first-order data
THe
In particular, rows and columns sum to 0.
This normalization can also be achieved as follows:
Let
The
Interpretation in this last table:
Compute the projection
Now, consider the easily understood functions
The space of these functions is a 100 dimensional subspace of
The entries in the table are the inner products
Summary observations
The data were to elect a president for the American Psychological
Association. Candidates 1 and 3 were clinicians
while candidates 4
and 5 were academicians
, two groups within the association with
somewhat divergent perspectives.
In the second-order table, we see a preference for candidates 1 & 3 witnessed
by the entry 376 corresponding to the entry for candidates
And we see a (slightly smaller) preference for candidates 4 and 5 witnessed by the entry
296 corresponding to the entry for candidates