% The characteristic polynomial of sparse zero-one matrices % Simon Coste - INRIA % Journées MAS 2021

# Eigenvalues of non-Hermitian matrices

> using LinearAlgebra > eigvals(randn(500,500))

# Example: random regular digraph

> using LinearAlgebra, Erdos > eigvals(random_regular_digraph(500, 3))

# My favourite example: Bernoulli, sparse

> using LinearAlgebra > eigvals(rand(500,500).<0.01)

$A_n$ = an $n \times n$ matrix whose entries are iid $\mathrm{Bernoulli}(d/n)$ entries.

# Reverse characteristic polynomial

$q_n (z) = \det(I_n - zA_n)$

The coefficients of $q_n(z)=1+\Delta_1z+\Delta_2z^2+...+\Delta_{n}z^{n}$ are

$\Delta_k = (-1)^k \frac{P_k(\mathrm{trace}(A_n^1), ..., \mathrm{trace}(A_n^k))}{k!},$

where the $P_k$ are polynomials.

# The limits of the traces of $A^k_n$

::: {.theorem}

For every $k$,

$(\mathrm{tr}(A_n^1), ..., \mathrm{tr}(A_n^k)) \xrightarrow[n \to \infty]{\mathrm{law}} (X_1, ... , X_k).$

where

$X_k := \sum_{\ell|k} \ell Y_\ell$

$(Y_\ell : \ell \in \mathbb{N}^*)$ = family of independent r.v., $Y_\ell \sim \mathrm{Poi}(d^\ell / \ell)$. :::

# The limits of the coefficients of $q_n$

$\Delta_k \to a_k = (-1)^k \frac{P_k(X_1, ... , X_k)}{k!}$

Let $F$ be the log-generating function of these random variables:

$F(z) = 1 + \sum_{k=1}^\infty a_k z^k$

:::{.theorem} Coefficients of $q_n$ $\to$ Coefficients of $F$ :::

Do we have stronger convergence than that?

# Weak convergence of analytic functions

::: {.theorem}

Shirai 2012

If $f_n$ is a sequence of random analytic functions in an open set $D$ and if

1. The coeffs of $f_n$ converge towards $(a_k)$

2. $f_n$ is tight in $D$

Then $f_n \to f$ where $f(z) = \sum a_k z^k$.

:::

# Tightness in holomorphic spaces

Let $f_n$ be a sequence of random analytic functions:

$f_n(z) = \sum_{k=0}^\infty a_{n,k}z^k.$

::: {.theorem}

If there is a $c$ such that

$\sup_n \mathbf{E}[|a_{n,k}|^2] \leqslant c r^k$

then $(f_n)$ is tight on $D(0,\sqrt{r})$.

:::

# Tightness of $(q_n)$

::: {.theorem} The sequence $q_n$ is tight in $D(0,\sqrt{1/d})$. :::

::: {.proof} Proof. We must bound the 2-norm of the coefficients of $q_n$, the $\Delta_k$.

We use $\Delta_k = \sum_{I \subset [n], |I|=k}\det(A(I))$ then develop $|\Delta_k|^2$.

We get a double sum of $\mathbf{E}[\det(A(I))\det(A(J))]$ with $I,J$ subsets of $[n]$.

The value of each summand depends on the size of $I\cap J$.

$\mathbf{E}[|\Delta_k|^2] = (n)_k (d/n)^k (1-d/n)^{k-1}(1 - kd/n -p + kd - k^2d/n) =O(d^k)$

:::

:::{.theorem} $q_n \to F$ as holomorphic functions on $D(0,d^{-1/2})$. :::

# Extras on $F$

$F(z) = \exp \left( -\sum_{k=1}^\infty X_k \frac{z^k}{k} \right) = \prod_{k=1}^\infty (1 - z^k)^{Y_k}$
• The radius of convergence inside the exp is $1/d$.

• The radius of convergence of $F$ is $1/\sqrt{d}$ and $F$ has one zero at $1/d$.

• $F$ has no other zeroes inside $D(0,1/\sqrt{d})$.

# Zeroes of $q_n$ => zeroes of $F$

:::{.theorem} The zeroes are continuous wrt weak convergence on $\mathbb{H}$. :::

Zeroes of $q_n$ inside $D(0,1/\sqrt{d})$ = inverse of eigenvalues of $A_n$ outside $D(0,\sqrt{d})$.

Asymptotically, $A_n$ has one eigenvalue close to $d$.

The other ones are smaller than $\sqrt{d}$.

# Friedman theorems everywhere

Can you have a short proof of Friedman's $2\sqrt{d-1}$-theorem?

1. Prove that the non-backtracking traces converge towards something [Dumitriu et al 2012]

2. Prove that $q_n$ is tight...

Bonne rentrée à tous !