## Seminars and Colloquia by Series

Series: PDE Seminar
Tuesday, February 26, 2019 - 15:00 , Location: Skiles 006 , Professor Hongjie Dong , Brown University , , Organizer: Xukai Yan

I will first give a short introduction of the Navier-Stokes equations, then review some previous results on theconditional regularity of solutions to the incompressible Navier-Stokes equations in the critical Lebesguespaces, and finally discuss some recent work which mainly addressed the boundary regularity issue.

Monday, February 25, 2019 - 15:30 , Location: Skiles 006 , Chris Davis , U Wisconsin Eau Claire , Organizer: Caitlin Leverson

Any knot in $S^3$ may be reduced to a slice knot by making some crossing changes. Indeed, this slice knot can be taken to be the unknot. We show that the same is true of knots in homology spheres, at least topologically. Something more complicated is true smoothly, as not every homology sphere bounds a smooth simply connected homology ball. We prove that a knot in a homology sphere is null-homotopic in a homology ball if and only if that knot can be reduced to the unknot by a sequence of concordances and crossing changes. We will show that there exist knot in a homology sphere which cannot be reduced to the unknot by any such sequence. As a consequence, there are knots in homology spheres which are not concordant to those examples produced by Levine in 2016 and Hom-Lidman-Levine in 2018.

Series: Other Talks
Monday, February 25, 2019 - 14:15 , Location: Skiles 005 , Prasad Tetali , Georgia Tech , Organizer: Greg Blekherman

Georgia Tech is leading the way in Creating the Next in higher education.In this talk I will present (1) My vision for ACO and (2) how my research relates naturally to ACO both where the A,C,O fields are going, and my own specific interests

Monday, February 25, 2019 - 14:00 , Location: Skiles 006 , Lisa Piccirillo , UT Austin , Organizer: Caitlin Leverson

Smooth simply connected 4-manifolds can admit second homology classes not representable by smoothly embedded spheres; knot traces are the prototypical example of 4-manifolds with such classes. I will show that there are knot traces where the minimal genus smooth surface generating second homology is not of the canonical type, resolving question 1.41 on the Kirby problem list. I will also use the same tools to show that Conway knot does not bound a smooth disk in the four ball, which completes the classification of slice knots under 13 crossings and gives the first example of a non-slice knot which is both topologically slice and a positive mutant of a slice knot.

Monday, February 25, 2019 - 11:15 , Location: Skiles 005 , Yun Yang , City Univ. NY , Organizer: Rafael de la Llave

The real world is inherently noisy, and so it is natural to consider the random perturbations of deterministic dynamical systems and seek to understand the corresponding asymptotic behavior, i.e., the phenomena that can be observed under long-term iteration. In this talk, we will study the random perturbations of a family of circle maps $f_a$. We will obtain, a checkable, finite-time criterion on the parameter a for random perturbation of $f_a$ to exhibit a unique, and thus ergodic, stationary measure.

Friday, February 22, 2019 - 16:00 , Location: Skiles 005 , Ian Jauslin , Princeton University , , Organizer: Federico Bonetto

Consider a metallic field emitter shaped like a thin needle, at the tip of which a large electric field is applied. Electrons spring out of the metal under the influence of the field. The celebrated and widely used Fowler-Nordheim equation predicts a value for the current outside the metal. In this talk, I will show that the Fowler-Nordheim equation emerges as the long-time asymptotic solution of a Schrodinger equation with a realistic initial condition, thereby justifying the use of the Fowler Nordheim equation in real setups. I will also discuss the rate of convergence to the Fowler-Nordheim regime.

Friday, February 22, 2019 - 12:00 , Location: Skile 006 , Cvetelina Hill , Georgia Tech , , Organizer: Cvetelina Hill
Friday, February 22, 2019 - 03:05 , Location: Skiles 246 , Longmei Shu , GT Math , Organizer: Jiaqi Yang

Isospectral reductions on graphs remove certain nodes and change the weights of remaining edges. They preserve the eigenvalues of the adjacency matrix of the graph, their algebraic multiplicities and geometric multiplicities. They also preserve the eigenvectors. We call the graphs that can be isospectrally reduced to one same graph spectrally equivalent. I will give examples to show that two graphs can be spectrally equivalent or not based on the feature one picks for the equivalence class.

Thursday, February 21, 2019 - 15:05 , Location: Skiles 006 , , University of Rochester , , Organizer: Michael Damron

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 families of semi-infinite geodesics in first- and last-passage percolation that come from Busemann functions. For d>=2, we describe the behavior of bi-infinite trajectories, and show that they carry an invariant measure. We also construct several examples displaying unexpected behavior. One of these examples lets us answer a question of C. Hoffman's from 2016. (joint work with Jon Chaika)

Wednesday, February 20, 2019 - 16:30 , Location: Skiles 006 , Michael Wigal , Georgia Tech , Organizer: Xingxing Yu

Let $g(n) = \max_{|T| = n}|\text{Aut}(T)|$ where $T$ is a tournament. Goldberg and Moon conjectured that $g(n) \le \sqrt{3}^{n-1}$ for all $n \ge 1$ with equality holding if and only if $n$ is a power of 3. Dixon proved the conjecture using the Feit-Thompson theorem. Alspach later gave a purely combinatorial proof. We discuss Alspach's proof and and some of its applications.