Groups and their Representations
Course timings: 9:30am to 11:00am, Wednesdays and Fridays
Location: Room 217
Jump to Lecture number 1, 2, 3, 4, 5, 6, 7, 8, 9, 9, 10, 11, 12, 13, 14, 15, 16
Breaking up $\QQ[\binom n2]$ into irreducibles (notes).
Homework
-
Assume that $k\leq 2n$.
Let
$$
V_k = \Big\{ f: \binom nk\to \QQ\mid \sum_{\{s\in \binom nk\mid s\supset t\}} f(s) = 0 \text{ for all } t\in \binom n{k-1}\Big\}.
$$
Compute the dimension of $V_k$.
-
Show that $V_k$ is an irreducible representation of $S_n$.
Construction of finite dimensional algebra in Sage, attempt at finding primitive idempotents in $\QQ[\binom n2]$ (Sage worksheet, notes)
Homework
Explicitly compute the primitive central idempotents in $\mathrm{End}_{S_n} \QQ[\binom n2]$.
Lecture 15
Structure constants for intertwining permutation representations (notes)
Homework
- Recall that the algebra $\CC[\binom n2]$ has basis $T_i:=T_{k_i}$ with $i = 0, 1, 2$.
Compute the products $T_iT_j$ in this algebra.
Find its primitive idempotents.
Lecture 14
Permutation Representation (notes)
Lecture 13
Schur's lemma (notes)
Homework
- Let $\rho$ be the two-dimensional representation of the cyclic group $C_n$ defined by $\rho(1) = \begin{pmatrix}\cos(2\pi/n) & \sin(2\pi/n) \\
-\sin 2\pi/n & \cos 2\pi/n\end{pmatrix}$.
Show that $\mathrm{End}_{C_4}\QQ^2$ is the field obtained by adjoining a primitive $n$th root of unity to $\QQ$.
- Let $f$ be a polynomial with distinct roots.
Let $A$ be a matrix with $f(A) = 0$.
Show that $A$ is diagonalizable.
- Determine whether or not the converse of Schur's lemma is true (assume $V$ finite dimensional over an algebraically closed field; does there exist a representation such that $\mathrm{End}_G V = K$, but $V$ is not simple.
GAP database for transitive subgroups, going from $S_n$ to $A_n$ in orbit problems, conjugacy classes in $A_n$ (notes).
Homework
- The GAP database of transitive groups shows two groups isomorphic to $S_5$ of degree $10$ (nos. 12 and 13).
Which of them is conjugate in $S_{10}$ to the subgroup from Lecture 11?
- Consider the action of $S_5$ on $\binom 52 \times \binom 52$ given by $w\cdot(x, y) = (w\cdot x, w\cdot y)$.
How many orbits does this action have?
Also, how many orbits does the the restriction of this action to $A_5$ have?
-
List representatives of the conjugacy classes of $A_5$ along with their cardinalities.
Transitive embedding of $S_5$ in $S_{10}$ motivated by five cubes in a dodecahedron. (notes)
Homework
- Let $V$ denote the space of all functions on the set of vertices of the regular dodecahedron such that
- the values add up to zero on each face
- the value at each vertex is equal to the value at its antipode.
With these definitions,
- compute the dimension of $V$.
- show that $V$, as a representation of the group of rigid motions which map the vertex-set of the dedecahedron onto itself (we know that this group is isomorphic to $A_5$), is irreducible.
Intertwining the two-dimensional representaion of $S_4$ with its twist by the sign character (notes).
Homweork
- Let $V_n$ be the $n-1$-dimensional space of vectors in $\QQ^n$ whose coordinates add up to $0$.
The symmetric group $S_n$ acts on this space by permuting the coordinates of these vectors:
$$
r_n(w)(x_1,\dotsc, x_n) = (x_{w(1)},\dotsc, x_{w(n)}).
$$
View $S_{n-1}$ as the subgroup of $S_n$ which permutes the elements $1,\dots,n-1$ of $\{1,\dotsc, n\}$.
Show that the restriction of $(r_n,V_n)$ is the sum of the trivial representation of $S_{n-1}$ and the representation $(r_{n-1}, V_{n-1})$.
-
Show that $(r_n, V_n)$ is an irreducible representation of $S_n$ for each positive integer $n$.
-
Show that $(r_n, V_n)$ is not isomorphic to $(r_n\otimes \epsilon, V_n)$ for any $n\geq 2$.
Detailed study of the embedding of $S_4$ into $S_6$ via its action on vertices of the octahedron,
Restriction of the two-dimensional representation of $S_4$ to $S_3$ (notes).
Interwtiners and invariant subspaces; a representation of $S_n$ coming from its action on $\binom n2$ (notes)
Homework
-
Let $V$ be the set of functions on the vertices of an octahedron whose sum, over any set of vertices which form a triangular face is $0$.
Choose two adjecent vertices and call them $1$ and $2$.
Let $v_1$ denote the unique element of $V$ whose value at $1$ is $1$ and value at $2$ is $0$.
Likewise, let $v_2$ denote the unique element of $V$ whose value at $1$ is $1$ and value at $1$ is $0$.
Then $v_1$ and $v_2$ form a basis of $V$.
Write down the matrices for the action of any set of generators $S_4$ (the group of rigid motions that fixes the set of vertices of the octahedron).
Conlude that $V$ is a simple representation.
-
With $T$ as above, and $\rho:S_4\to GL(V)$ denoting the action of $S_4$ on it, find a linear isomorphism $T:V\to V$ such that
$$\rho(w)T = \epsilon(w)T\rho(w).$$
-
Let $V$ be the space of all functions from the set of vertices of an icosahedron to $\QQ$, the sum of whose values on each face is $0$.
What is $\dim V$?
Note: You may want to take this into account before you shout out your answer.
Symmetries of the dodecahedral graph, definition of a representation (notes)
Homework
-
Show that the action of the group of rigid motions of of the dodecahedron on its set of five embedded cubes defines an isomorphism from this group onto $A_5$.
-
Consider the representation of $S_4$ on the three-dimensional subspace $V$ of vectors in $\mathbf Q^4$ whose coordinates add up to $0$.
Is there proper subspace of $V$ that is mapped to itself (i.e., is invariant) under this action?
Symmetries of graphs (notes).
Homework
-
Show that the automorphism group of the graph on the top of the first page of the notes is isomorphic to the dihedral group with eight elements.
-
Decide whether or not the automorphism group of the cube graph is isomorphic to $S_4\times C_2$ (here $C_2$ is the cyclic group with two elements).
Symmetries of Platonic solids, duality of Platonic solids.
Homework
- Let $G$ be the group of symmetries of the cube.
Let $\Gamma$ be the graph whose vertices are the corners of the cube, and edges are the edges of the cube.
Let $\mathrm{Aut}(\Gamma)$ denote the group of bijections of the vertices of $\Gamma$ which preserve edge relations. What is the order of this group? What is the relationship between this group and $G$?
- Repeat the previous exercise with the cube replaced by the tetrahedron, and by the dodecahedron.
- Show that the group of rigid motions that preserve the dodecahedron is isomorphic to $A_5$. Click here for a hint; also try using the dodecahedral graph to make your work easier.
Groups preserving point configurations (notes).
Homework
-
Consider the following algorithm:
- Start with the permutation $w = 12\dotsb n$.
- As $i$ goes from $1$ to $n$ do the following:
- swap the element $w_i$ with a random element from $w_{i+1},\dotsc, w_n$.
This is a minor change from Problem 3 from Lecture 1 (there $w_i$ was swapped with a random element from $w_i, \dotsc, w_m$).
What is the resulting probability distribution on $S_n$?
-
Let $\langle \vec x, \vec y\rangle$ denote the dot product of vectors in $\RR^3$, and $\|\vec x\|^2 = \langle \vec x, \vec x \rangle$.
Show that if $\|A \vec v\| = \|\vec v\|$ for each $\vec v \in \RR^3$, then $\langle A\vec v, A\vec w\rangle = \langle \vec v, \vec w\rangle$ for all $\vec v, \vec w\in \RR^3$.
-
We have seen that $G_{(0,0,0)}$ the group of rigid motions of space which fix the origin is $SO(3)$, the group of orthogonal matrices with determinant one.
Find a nice description of the group $G_{(1,0,0)}$, the group of rigid motions which fix the first coordinate vector.
-
Think of $\RR^3$ as the subspace of $\RR^4$ consisting of vectors whose coordinates add up to $1$.
In this space, the coordinate vectors $(1,0,0,0)$, $(0, 1, 0, 0)$, $(0, 0, 1, 0)$ and $(0, 0, 0, 1)$ form the vertices of a regular tetrahedron.
Use this idea to write down the coordinates of four points in $\RR^3$ which form the corners of a regular tetrahedron.
-
Express the element $(12)(34)$ of $S_4$ as a composition of $3$-cycles.
-
Does there exist a configuration $C$ of four points in $\RR^3$ such that the group of rigid motions which fix $C$ has $24$ elements?
-
Is the group of rigid motions that fix the set of vertices of a cube isomorphic to the symmetric group? Note: the vertices of a cube can be taken to be the eight vectors of the form $(\pm 1, \pm 1, \pm 1)$.
-
Is the group of rigid motions that fix the set of vertices of a cube isomorphic to the group of rigid motions of a regular octahedron? Note: the vertices of a regular octahedron can be taken to be the three coordinate vectors in $\RR^3$ and their negatives.
Random permutations revisited; distribution of product of independent distributions is given by permutations; Knuth shuffling and a canonical form.
Homework
- Find the Knuth shuffle decomposition of the permutation $w = 7653124$, i.e., express this permutation as a product
$$
(i_1 j_1)(i_2 j_2)\dotsb (i_k j_k),
$$
where $1\leq i_1< i_2 \dotsb < i_k\leq n$, and $i_r< j_r$ for each $1\leq r\leq k$ (here $n= 7$).
-
Find an algorithm to write any permutation as its Knuth shuffle decomposition (see pervious question for definition of Knuth shuffle decomposition).
Permutations: Generating function for inversions, cycle decompositions (notes).
Homework
- Let $w\in S_n$ be the permutation for which $w(i) = j$ and $w(j) = i$ for some $1\leq i < j \leq n$. Compute $\mathrm{inv}(w)$.
- Recall that a polynomial $a_0 + a_1t + \dotsb + a_Nt^N$ with positive coefficients of degree $N$ is called symmetric unimodal if $a_i = N_{N-i}$ for each $0\leq i\leq N$, and $a_i\leq a_{i+1}$ for $0\leq i \lt N/2$.
Show that a product of symmetric unimodal polynomials is symmetric unimodal.
Conculde that the distribution of the inversion number statistic on $S_n$ is unimodal.
- The rank of a permutation $w\in S_n$ is defined to be its position in the list of all permutations written out in lexicographic order.
For example, $231$ has rank $4$.
Compute the rank of the permutation $64312875$ without using a computer.
Check your answer with Sage.
- How many permutations of $8$ have cycle type $(3, 2, 2, 1)$?
Permutations: lexicographic order, enumeration, random sampling, fixed points, descents, inversions. First steps with Sage.
Homework
- Suppose that $\alpha, \beta: 2^{\mathbf n}\to \mathbf C$ satisfy the identity:
$$
\alpha(I) = \sum_{J\supset I} (-1)^{|J|-|I|}\beta(J) \text{ for all } I, J \subset \mathbf n.
$$
Show that
$$
\beta(I) = \sum_{J\supset I} \alpha(J) \text{ for all } I, J \subset \mathbf n.
$$
-
Suppose that a vector $\mathbf x \in [0, 1]^n$ is chosen at random, and then the coordinates of this vector, taken in increasing order are replaced by the integers $1,2,\dotsc, n$, then the probability that the result is a given permutation $w$ of size $n$ is $1/n!$.
-
Consider the following algorithm:
- Start with the permutation $w = 12\dotsb n$.
- As $i$ goes from $1$ to $n$ do the following:
- swap the element $w_i$ with a random element from $w_i,\dotsc, w_n$.
Show that the probability of ending up with any given permutation is $1/n!$.