While in graduate school, I was quite fascinated by the various counting and book-keeping techniques in graph theory and group theory. One nice example is Burnside’s lemma, which helps to count the number of different necklaces that can be made out of say 5 black stones and 4 white stones. I will write a separate post for it when I get some free time.

On the other hand, there are many theorems and techniques that I don’t have much feelings for, mostly because I don’t see the applications. One prominent example is Schur’s lemma, which states that

given an irreducible matrix representation of a group, if a matrix commutes with all group members, then it is basically the identity.

The lemma is quite intuitive by itself. At the time, I actually wondered why people bothered to write it down and even name it.

Many years later, my colleague at medical school needed to evaluate some expressions for her NeuroImage paper (in case you do not know, NeuroImage is a not-so-technical journal that publishes pretty brain images and bar plots from statistical analysis). To my surprise, her problem was in a perfect set-up for Schur’s lemma and the result was even not too trivial (we didn’t end up with identity).

In this post, I will demonstrate this application.

## simple examples

First let me show you some concrete examples to get some sense of Schur’s lemma.

#### example 1

The symmetric group $$S_2$$ has the following matrix representation

$I = \begin{bmatrix} 1 & 0 \\ 0 & 1\end{bmatrix},\quad p =\begin{bmatrix} 0 & 1 \\ 1 & 0\end{bmatrix}$

You can check that any 2-by-2 diagonal matrix commutes with both $$I$$ and $$p$$. This means that the matrix representation above is not irreducible. In fact, there are 2 scalar irreducible representations for $$S_2$$. One of them is the trivial representation where all group elements are represented by $$1$$. In the other one, $$I$$ is represented by $$1$$ and $$p$$ is represented by $$-1$$.

This brings up a question of whether a given representation is irreducible. Fortunately, it can be tested easily using the orthogonality of irreducible characters.

For irreducible representations, we have

$\left<\chi_R, \chi_R \right> = \frac{1}{|G|} \sum_{g\in G} \chi_R(g)\overline{\chi_R(g)}= 1$

where $$\chi_R(g)$$ is the character (matrix trace) of the group element $$g$$.

For $$S_2$$, this inner product gives 2, which tells us the 2-by-2 matrix representation is reducible, and it can be decomposed into 2 irreducible representations.

#### example 2

Let’s look at a 2D representation of the point group $$C_{3v}$$ (It is the symmetry of Ammonia molecule). There are 6 group elements and can be generated by a 3-fold rotation $$C_3$$ and a mirror operation $$\sigma_v$$:

$C_3 = \begin{bmatrix} -\frac{1}{2} & -\frac{\sqrt{3}}{2} \\ \frac{\sqrt 3}{2} & -\frac1{2} \end{bmatrix}, \quad \sigma_v = \begin{bmatrix} 1 & 0\\ 0 & -1 \end{bmatrix}$

The explicit matrix form of all 6 elements can be found in this page.

You can convince yourself that the only 2-by-2 matrix commuting with both of them is a scalar times $$I$$. You can also check that the orthogonality of irreducible characters holds for this representation. This is the scenario where Schur’s lemma applies.

(This example is not as simple as I hoped. Please let me know if you know a simpler 2D irreducible representation.)

#### example 3

Note that the matrix representation needs to be irreducible in order to apply Schur’s lemma. Suppose we have the following 4-element group

$I=\begin{bmatrix} 1&0&0&0\\0&1&0&0\\0&0&1&0\\0&0&0&1\end{bmatrix}, \quad p_1=\begin{bmatrix}0&1&0&0\\1&0&0&0\\0&0&1&0\\0&0&0&1\end{bmatrix}, \quad p_2=\begin{bmatrix}1&0&0&0\\0&1&0&0\\0&0&0&1\\0&0&1&0\end{bmatrix}, \quad p_3=\begin{bmatrix}0&1&0&0\\1&0&0&0\\0&0&0&1\\0&0&1&0\end{bmatrix}.$

This is not an irreducible representation due to its block-diagonal structure. As a result, the matrices that commutes with all four group members are not all proportional to the identity matrix. For example, the one below also commutes with all four elements.

$\begin{bmatrix} 1&0&0&0\\0&1&0&0\\0&0&2&0\\0&0&0&2\end{bmatrix}$

## application in averaging tensors

### the goal

In our NeuroImage paper, we would like to evaluate two ensemble averages of the susceptibility tensor $$\chi$$ over all possible rotations

\begin{align} \left<\chi \right>_1 &= \sum_i R_i \chi R_i^{-1} \\ \left<\chi \right>_2 &= \sum_i Q_i \chi Q_i^{-1}. \end{align}

Here $$R_i$$ and $$Q_i$$ denote three dimensional rotations and the summation runs over all possible rotations of the corresponding types. I write the averaging as discrete summation for notational simplicity: rotations form Lie groups and should be parametrized by continuous variables.

The rotations $$Q_i$$’s have the special property that they rotate the z component of any vector into the x-y plane and thus can be conveniently parametrized with three Euler angles $$(\theta_1, \theta_2=\pi/2, \theta_3)$$ in the ZXZ convention. More explicitly, we have

\begin{align} Q_i = R_z(\theta_1) R_x(\pi/2) R_z(\theta_3). \end{align}

### the physical motivation

The summation over rotations $$R_i$$ and $$Q_i$$ may appear a bit abrupt to you.

What happens is that we know the magnetic susceptibility tensor (a 3-by-3 matrix) for each molecule and we want to know the averaged tensor for an ensemble of the molecules.

For $$\left<\chi\right>_1$$, we assume the molecules are randomly oriented, i.e., each one of them is pointing to a random direction and is then parametrized as $$R_i\chi R_i^{-1}$$.

For $$\left<\chi\right>_2$$, we assume the molecules are organized in a special way. They firstly form rings in two dimension, then the rings form cylinders. This model is motivated by the myelin sheath in white matter tracks.

More background information of the project can be found in our NeuroImage paper.

### the evaluations

To evaluate the first summation, we note that $$\left<\chi\right>_1$$ commutes with any rotation $$R_i$$ in SO(3). According to Schur’s lemma, $$\left<\chi\right>_1$$ is proportional to identity matrix. Also note rotations preserve matrix trace, thus we have

\begin{align} \left<\chi\right>_1 = \frac{\chi_{11} + \chi_{22} + \chi_{33}}{3} \ I, \end{align}

where $$I$$ is the identity matrix.

The second summation can be done similarly. Note the representation is not irreducible in this case. To be more explicit,

\begin{align} \left<\chi\right>_2 = &\sum_{\theta_1,\theta_3} R_z(\theta_1) R_x(\pi/2) R_z(\theta_3) \chi R_z^{-1}(\theta_3) R_x^{-1}(\pi/2) R_z^{-1} (\theta_1) \\ = & \sum_{\theta_1} R_z(\theta_1) R_x(\pi/2) \begin{bmatrix} \frac{\chi_{11}+\chi_{22}}{2} & 0 & 0 \\ 0 & \frac{\chi_{11}+\chi_{22}}{2} & 0 \\ 0 & 0 & \chi_{33} \end{bmatrix} R_x^{-1}(\pi/2) R_z^{-1} (\theta_1) \\ =& \sum_{\theta_1}R_z(\theta_1) \begin{bmatrix} \frac{\chi_{11}+\chi_{22}}{2} & 0 & 0 \\ 0 & \chi_{33} & 0\\ 0 & 0 & \frac{\chi_{11}+\chi_{22}}{2} \end{bmatrix} R_z^{-1} (\theta_1) \\ =& \begin{bmatrix} \frac{\chi_{11}+\chi_{22}}{4}+ \frac{\chi_{33}}{2} & 0 & 0 \\ 0 & \frac{\chi_{11}+\chi_{22}}{4}+ \frac{\chi_{33}}{2} & 0\\ 0 & 0 & \frac{\chi_{11}+\chi_{22}}{2} \end{bmatrix} \end{align}

## final thoughts

You can’t connect the dots looking forward; you can only connect them looking backwards. So you have to trust that the dots will somehow connect in your future. — Steve Jobs

It is nice that some random stuff from the past, especially one that I considered to be trivial and useless, can actually be put in use in a totally unexpected way. I am looking forward to seeing how my other random dots are going to connect in the future.