# Probability Seminar

The seminar is jointly sponsored by Temple and Penn. The organizers are Brian Rider and Atilla Yilmaz (Temple), and Jian Ding and Robin Pemantle (Penn).

Talks are Tuesdays 3:00 - 4:00 pm and are held either in Wachman Hall (Temple) or David Rittenhouse Lab (Penn) as indicated below.

For a chronological listing of the talks, click the year above.

• Tuesday December 4, 2018 at 15:00, Temple (Wachman 617)
TBA

Pascal Maillard, Orsay/CRM

• Tuesday November 27, 2018 at 15:00, Temple (Wachman 617)
TBA

Sunder Sethuraman, University of Arizona

• Tuesday November 13, 2018 at 15:00, Penn (DRL 4C8)
TBA

Abram Magner, Purdue University

• Tuesday October 30, 2018 at 15:00, Temple (Wachman 617)
TBA

Anirban Basak, Tata Institute

• Tuesday October 23, 2018 at 15:00, Penn (DRL 4C8)
TBA

Julian Gold, Northwestern University

• Tuesday October 9, 2018 at 15:00, Temple (Wachman 617)
The coin turning walk and its scaling limit

Janos Englander, CU Boulder

Given a sequence of numbers $p_n ∈ [0, 1]$, consider the following experiment. First, we flip a fair coin and then, at step $n$, we turn the coin over to the other side with probability $p_n, n > 1$, independently of the sequence of the previous terms. What can we say about the distribution of the empirical frequency of heads as $n \to \infty$? We show that a number of phase transitions take place as the turning gets slower (i.e. $p_n$ is getting smaller), leading first to the breakdown of the Central Limit Theorem and then to that of the Law of Large Numbers. It turns out that the critical regime is $p_n = \textrm{const}/n$. Among the scaling limits, we obtain Uniform, Gaussian, Semicircle and Arcsine laws. The critical regime is particularly interesting: when the corresponding random walk is considered, an interesting process emerges as the scaling limit; also, a connection with Polya urns will be mentioned. This is joint work with S. Volkov (Lund) and Z. Wang (Boulder).

• Tuesday October 2, 2018 at 15:00, Temple (Wachman 617)
TBA

Firas Rassoul-Agha, University of Utah

• Tuesday September 25, 2018 at 15:00, Penn (DRL 4C8)
Zeros of polynomials, the distribution of coefficients, and a problem of J.E. Littlewood

Julian Sahasrabudhe, Cambridge University

While it is an old and fundamental fact that every (nice enough) even function $f : [-\pi,\pi] \rightarrow \mathbb{C}$ may be uniquely expressed as a cosine series $f(\theta) = \sum_{r \geq 0 } C_r\cos(r\theta),$ the relationship between the sequence of coefficients $(C_r)_{r \geq 0 }$ and the behavior of the function $f$ remains mysterious in many aspects. We mention two variations on this theme. First a more probabilistic setting: what can be said about a random variable if we constrain the roots of the probability generating function? We then settle on our main topic; a solution to a problem of J.E. Littlewood about the behavior of the zeros of cosine polynomials with coefficients $C_r \in \{0,1\}$.

• Tuesday September 18, 2018 at 15:00, Temple (Wachman 617)
Stationary coalescing walks on the lattice

Arjun Krishnan, University of Rochester

Consider a measurable dense family of semi-infinite nearest-neighbor paths on the integer lattice in d dimensions. If the measure on the paths is translation invariant, we completely classify their collective behavior in d=2 under mild assumptions. We use our theory to classify the behavior of semi-infinite geodesics in random translation invariant metrics on the lattice; it applies, in particular, to first- and last-passage percolation. We also construct several examples displaying unexpected behaviors. (Joint work with Jon Chaika.)

• Tuesday September 11, 2018 at 15:00, Temple (Wachman 617)
The Sine-beta process: DLR equations and applications

Thomas Leblé, NYU Courant

One-dimensional log-gases, or Beta-ensembles, are statistical physics toy models finding their incarnation in random matrix theory. Their limit behavior at microscopic scale is known as the Sine-beta process, its original description involves systems of coupled SDE’s.
We give a new description of Sine-beta as an « infinite volume Gibbs measure », using the Dobrushin-Lanford-Ruelle (DLR) formalism, and use it to prove the “rigidity” of the process, in the sense of Ghosh-Peres. If time permits, I will mention another application to the study of fluctuations of linear statistics. Joint work with David Dereudre, Adrien Hardy, and Mylène Maïda.

• Tuesday September 4, 2018 at 15:00, Penn (DRL 4C8)
In between random walk and rotor walk in the square lattice

Swee Hong Chen, Cornell

How much randomness is needed to prove a scaling limit result? In this talk we consider this question for a family of random walks on the square lattice. When the randomness is turned to the maximum, we have the symmetric random walk, which is known to scale to a two-dimensional Brownian motion. When the randomness is turned to zero, we have the rotor walk, for which its scaling limit is an open problem. This talk is about random walks that lie in between these two extreme cases and for which we can prove their scaling limit. This is a joint work with Lila Greco, Lionel Levine, and Boyao Li.