# Remarks on Lecture 1

Today we started studying the spectrum of the combinatorial Laplacian on a graph $G=(V,E)$. This is the operator $L_G : \mathbb R^V \to \mathbb R^V$ given by

${\displaystyle L_G f(x) = \mathrm{deg}(x) f(x) - \sum_{y \sim x} f(y)\,.}$

We saw that $L_G$ has a spectrum of the form $0 = \lambda_1 \leq \lambda_2 \leq \cdots \leq \lambda_n$, and that the number of connected components in $G$ is precisely the multiplicity of $\lambda_1$.

I encourage you to look at the first four lectures of Spielman’s course for a refresher.

### Exercises

The best way to learn a subject is to get your hands dirty. Here are some optional exercises to help build your intuition.

Recall that for a function $f : V \to \mathbb R$, we define its Rayleigh quotient by

$\displaystyle \mathcal R_G(f) = \frac{\sum_{x \sim y} (f(x)-f(y))^2}{\sum_{x \in V} f(x)^2}.$

If $P_n$ is the path graph on n vertices, then as we argued today, we have $\lambda_k \asymp \left(\frac{k}{n}\right)^2$.

1. Prove that $\lambda_2 \asymp \frac{1}{n^2}$ by showing that (a) there exists a map $f : V \to \mathbb R$ with $f \perp \mathbf{1}$ and $\mathcal R_G(f) \lesssim \frac{1}{n^2}$ and (b) for any such map with $f \perp \mathbf{1}$, we have $\mathcal R_G(f) \gtrsim \frac{1}{n^2}$. We did this in class, but it will help to reconstruct the proof on your own.

2. Try to prove that $\lambda_k \lesssim \left(\frac{k}{n}\right)^2$ by exhibiting an explicit subspace of test functions
that achieves this bound. It may help to use the Courant-Fischer min-max principle
which says that

$\displaystyle \lambda_k = \min_{S \subseteq \mathbb R^V} \max_{0 \neq f \in S} \mathcal R_G(f),$

where the minimum is over all k-dimensional subspaces $S$.

[Hint: Have your subspace be the span of k functions with disjoint supports, where the support of a function $f : V \to \mathbb R$ is $\mathrm{supp}(f) = \{ x \in V : f(x) \neq 0 \}$.]

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## 2 thoughts on “Remarks on Lecture 1”

1. Cyrus Rashtchian says:

For (1), is there a high level way to see the lower bound for R(f)? I follow the proof given in class but somehow it’s a hard proof to remember because it seems very arithmetic.

Specifically, why would you think to sum over choices for a fixed vertex, and why does it make sense to employ cauchy-schwarz?

2. Let’s normalize so that $f : V \to \mathbb R$ satisfies $\frac{1}{n} \sum_{v \in V} f(v)^2 = 1$. We also know that $\sum_{v \in V} f(v) = 0$.

These two facts together suggests that f has to have a large variance in values. For instance, it is possible to prove from these facts that $\frac{1}{n^2} \sum_{u,v \in V} (f(u)-f(v))^2 = 2$. Instead of using this sum over pairs, you can just fix a vertex. This is what we did in class, and it captures the same effect.

Now you want to say that any function which varies a lot (in this sense) must have to vary somewhat along edges. Consider the endpoints of the path $v_1$ and $v_n$. We know that the edges in between much “pay” for the stretch $(f(v_1)-f(v_n))^2$. The cheapest way to do this (because we’re looking at a sum of squares) is for every edge to be of length $|f(v_1)-f(v_n)|/(n-1)$. Cauchy-Schwarz is the formal way to prove that this is “cheapest.”