Top eigenvalues of random trees

Let $T_n$ be a uniformly random tree with vertex set $[n]={1,…,n}$. Let $Delta_n$ be the largest vertex degree in $T_n$ and let $\lambda_n$ be the largest eigenvalue of $T_n$. We show that $|\lambda_n-\sqrt{\Delta_n}| \to 0$ in probability as $n \to \infty$. The key ingredients of our proof are (a) the trace method, (b) a rewiring lemma that allows us to “clean up” our tree without decreasing its top eigenvalue, and© some careful combinatorial arguments.

This is extremely slow joint work with Roberto Imbuzeiro Oliveira and Gabor Lugosi, but we hope to finally finish our write-up in the coming weeks.