## Departmental PhD Thesis Exam – Duncan Dauvergne

Everyone is welcome to attend.  Refreshments will be served in the Math Lounge before the exam.

Monday, March 25, 2019
10:10 a.m.
BA6183

PhD Candidate:  Duncan Dauvergne
Supervisor:   Balint Virag
Thesis title:  Random sorting networks, the directed landscape, and random polynomials

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The first part of this thesis is on random sorting networks. A sorting network is a shortest path from $12 \cdots n$ to $n \cdots 21$ in the Cayley graph of the symmetric group $S_n$ generated by adjacent transpositions. We prove that in a uniform random $n$-element sorting network $\sigma^n$, all particle trajectories are close to sine curves with high probability. We also find the weak limit of the time-$t$ permutation matrix measures of $\sigma^n$. As a corollary, we show that if $S_n$ is embedded into $\mathbb{R}^n$ via the map $\tau \mapsto (\tau(1), \tau(2), \dots \tau(n))$, then with high probability, the path $\sigma^n$ is close to a great circle on a particular $(n-2)$-dimensional sphere. These results prove conjectures of Angel, Holroyd, Romik, and Vir\’ag. To prove these results, we find the local limit of random sorting networks and prove that the local speed distribution is the arcsine distribution on $[-\pi, \pi]$.

The second part of this thesis is on last passage percolation. The conjectured limit of last passage percolation is a scale-invariant, independent, stationary increment process with respect to metric composition. We prove this for Brownian last passage percolation. We construct the Airy sheet and characterize it in terms of the Airy line ensemble. We also show that the last passage geodesics converge to random functions with H\”older-$2/3^-$ continuous paths. To prove these results, we develop a new probabilistic framework for understanding the Airy line ensemble.

The third part of this thesis is on random sums of orthonormal polynomials. Let $G_n = \sum_{i=0}^n \xi_i p_i$, where the $\xi_i$ are i.i.d. non-degenerate complex random variables, and $\{p_i\}$ is a sequence of orthonormal polynomials with respect to a regular measure $\tau$ supported on a compact set $K$. We show that the zero measure of $G_n$ converges weakly almost surely to the equilibrium measure of $K$ if and only if $\mathbb{E}\log(1 + |\xi_0|) < \infty$. We also show that the zero measure of $G_n$ converges weakly in probability to the equilibrium measure of $K$ if and only if $\mathbb{P}(|\xi_0| > e^n) = o(n^{-1})$. Our methods also work for more general sequences of asymptotically minimal polynomials in $L^p(\tau)$, where $p \in (0, \infty]$.

A copy of the thesis can be found here: MainThesisPhD