# Lecture 7-8

We discussed combinatorial and spectral notions of graph expansion and sketched why a random d-regular graph is an expander for d large enough. Then I presented Margulis’ expander construction and the Gabber-Galil analysis. To end, we proved that expanders require logarithmic distortion to embed into any Euclidean space.

Here is a modified analysis of the Margulis expanders.

# Lecture 6

Here are the slides, and here’s a proof of the crossing number inequality.

From this paper, we know that for an n-vertex planar graph with maximum degree ${\Delta}$, we have for every k, the eigenvalue bound

${\lambda_k \leq O(\Delta k/n)}$

Open question: What about the eigenvalue gaps? I conjecture that the stronger bound ${\lambda_{k+1} - \lambda_k \leq O(\Delta/n)}$ holds for every k. I believe it is open even for k=2.

# Homework #1

This homework is due in two weeks on Wednesday, April 18th. You make work in small groups of 2 or 3. You should reference any discussions with others, books, lecture notes, or other resources that you use in coming up with a solution.

[Reminder: The notation ${A \lesssim B}$ is the same as ${A \leq O(B)}$ and ${A \asymp B}$ is defined as the conjuniction of ${A \lesssim B}$ and ${B \lesssim A}$.]

### Problems

1. Let ${A}$ be the adjacency matrix of a ${d}$-regular graph ${G}$. Prove that ${G}$ is bipartite if and only if ${A}$ has ${-d}$ as an eigenvalue.
2. Consider the ${L \times L}$ two-dimensional grid graph ${G_{L,L}}$. Let ${n=L^2}$. Prove that

$\displaystyle \lambda_2(G_{L,L}) \asymp \frac{1}{n},$

where ${\lambda_2}$ refers to the second eigenvalue of the combinatorial Laplacian. (Prove this without using the Spielman-Teng theorem for planar graphs.) For the lower bound, it may help to remember that ${\lambda_2(P_L) \asymp \frac{1}{L^2}}$, where ${P_L}$ is the path on ${L}$ vertices.

3. Let ${n=2^h-1}$ for some integer ${h \geq 1}$. Prove that if ${T_h}$ is the complete binary tree of height ${h}$, we have

$\displaystyle \lambda_2(T_h) \asymp \frac{1}{n},$

where again ${\lambda_2}$ refers to the combinatorial Laplacian.

4. Let ${G}$ be an arbitrary graph. Recall that

$\displaystyle h_G = \min_{|S| \leq n/2} \frac{|E(S)|}{|S|}.$

Let ${\lambda_2}$ be the second eigenvalue of the combinatorial Laplacian on ${G}$.

Prove that,

$\displaystyle \lambda_2 = \min_{f : V \rightarrow \mathbb R} \max_{c \in \mathbb R} \frac{\sum_{u \sim v} |f(u)-f(v)|^2}{\sum_{u \in V} |f(u)-c|^2}\,,$

and

$\displaystyle h_G = \min_{f : V \rightarrow \mathbb R} \max_{c \in \mathbb R} \frac{\sum_{u \sim v} |f(u)-f(v)|}{\sum_{u \in V} |f(u)-c|}\,,$

where both minimums are over non-constant functions.

5. Let ${G}$ be a ${d}$-regular graph and set ${M = \frac12(I+\frac{1}{d} A)}$, where ${A}$ is the adjacency matrix of ${G}$. Let ${\tau = \tau_{\infty}}$ be defined as in the notes for lecture 3. Let ${\mu_1, \mu_2, \ldots, \mu_n}$ be the eigenvalues of ${M}$. Prove that

$\displaystyle \sum_{i=1}^n \mu_i^{2\tau} \leq O(1)\,,$

by evaluating ${\mathrm{tr}(M^{2\tau})}$ in two different ways.

# Remarks on Lecture 4

You can see notes for this lecture here. In the next lecture, we will cover a different approach based on these notes.

# Remarks on Lecture 3

Today we saw the connection between mixing times of random walks and the second eigenvalue of the normalized Laplacian. Recall that ${G=(V,E,w)}$ is an ${n}$-vertex, weighted graph.

First, we introduced the lazy walk matrix,

$\displaystyle W_G = \frac{1}{2} (I + A_G D_G^{-1})\,,$

which is related to the normalized Laplacian via,

$\displaystyle W_G = I - \frac{1}{2} D_G^{1/2} \mathcal{L}_G D_G^{-1/2}\,.$

In particular, if we arrange the eigenvalues of ${W_G}$ as ${1=\beta_1 \geq \beta_2 \geq \cdots \geq \beta_n}$, then

$\displaystyle \beta_i = 1 - \frac{\lambda_i}{2}\,,$

where ${\lambda_i}$ is the ${i}$th smallest eigenvalue of ${\mathcal{L}_G}$. Furthermore, the eigenfunctions of ${W_G}$ are given by ${g = D_G^{1/2} f}$, where ${f}$ is an eigenfunction of ${\mathcal L_G}$.

The eigenfunctions of ${W_G}$ are orthogonal with respect to the inner product

$\displaystyle (f,g) = \sum_{u \in V} \frac{1}{w(u)} f(u) g(u)\,.$

We will refer to this Hilbert space as ${\ell^2(1/w)}$. Recall that ${\|f\|_{\ell^2(1/w)} = \sqrt{(f,f)} = \sqrt{\sum_{v \in V} \frac{1}{w(v)} f(v)^2}}$.

If we define the stationary distribution ${\pi : V \rightarrow [0,1]}$ by

$\displaystyle \pi(u) = \frac{w(u)}{w(V)} = \frac{w(u)}{2 w(E)},$

then ${W_G \pi = \pi}$.

Let ${f_1, f_2, \ldots, f_n}$ be an ${\ell^2(1/w)}$-orthornormal system of eigenvectors for ${W_G}$. Now fix an initial distribution ${p_0 : V \rightarrow [0,1]}$ and write,

$\displaystyle p_0 = \sum_{i=1}^n \alpha_i f_i\,.$

Then we have,

$\displaystyle p_t = W^t p_0 = \pi + \sum_{i > 1} \alpha_i \left(1-\frac{\lambda_i}{2}\right)^t f_i\,.$

In particular,

$\displaystyle \|p_t - \pi\|_{\ell^2(1/w)}^2 = \sum_{i > 1} \alpha_i^2 \left(1-\frac{\lambda_i}{2}\right)^{2t} \leq \left(1-\frac{\lambda_2}{2}\right)^{2t} \sum_{i > 1} \alpha_i^2 \leq \left(1-\frac{\lambda_2}{2}\right)^{2t} \|p_0\|_{\ell^2(1/w)}^2\,.$

Let ${\pi_* = \min_{u \in V} \pi(u)}$ be the minimum stationary measure. A particularly strongly notion of mixing time is given by the following definition.

$\displaystyle \tau_{\infty} = \sup_{p_0} \min \left\{ t : \|p_t - \pi\|_{\infty} < \frac{\pi_*}{4} \right\}\,$

where the supremum is taken over all initial distributions ${p_0}$. By time ${\tau_{\infty}}$, the distribution ${p_t}$ is pointwise very close to the stationary distribution ${\pi}$.

Let ${w^* = \max_{u \in V} w(u)}$ and ${w_* = \min_{u \in V} w(u)}$. Using our preceding calculation, we have

$\displaystyle \|p_t - \pi\|_{\infty} \leq \sqrt{w^*} \|p_t - \pi\|_{\ell^2(1/w)} \leq \sqrt{w^*} \left(1-\frac{\lambda_2}{2}\right)^{t} \|p_0\|_{\ell^2(V,w)} \leq \left(1-\frac{\lambda_2}{2}\right)^{t} \sqrt{\frac{w^*}{w_*}}\,.$

Observe that ${\frac{w^*}{w_*} \leq \frac{1}{\pi_*}}$, so if we now choose ${t \asymp \frac{1}{\lambda_2} \log \frac{1}{\pi_*}}$ appropriately, then

$\displaystyle \|p_t - \pi\|_{\infty} < \frac{\pi_*}{4},$

yielding our main result:

$\displaystyle \tau_{\infty} \lesssim \frac{1}{\lambda_2} \log \frac{1}{\pi_*}\,.$

One can also proof that ${\tau_{\infty} \gtrsim \frac{1}{\lambda_2}}$ (see the exercises). In general, we will be concerned with weaker notions of mixing. We’ll discuss those when they arise.

### Excersies

Prove that ${\tau_{\infty} \gtrsim \frac{1}{\lambda_2}}$ for any graph G.

# Agenda

Our agenda for the next few weeks looks something like this:

1. The discrete Cheeger inequality
2. The connection between ${\lambda_2}$ and the mixing time of random walks
3. Planar separators, eigenvalues, and circle packings
4. Expander graphs and rapid mixing
5. The small-set expansion problem and the Unique Games Conjecture
6. Higher-order Cheeger inequalities
7. Cheeger inequalities for very large eigenvalues (Arora-Barak-Steurer)
8. Mixing times and the spectral profile