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Series: Geometry Topology Seminar

We use Manolescu's Pin(2)-equivariant Floer homology to study homology cobordisms among Seifert spaces. In particular, we will show that the subgroup of the homology cobordism group generated by Seifert spaces admits a \mathbb{Z}^\infty summand. This is joint work with Irving Dai.

Monday, October 2, 2017 - 13:55 ,
Location: Skiles 005 ,
Weilin Li ,
University of Maryland, College Park ,
wl298@math.umd.edu ,
Organizer: Wenjing Liao

We formulate
super-resolution as an inverse problem in the space of measures, and
introduce a discrete and a continuous model. For the discrete model, the
problem is to accurately recover a sparse high dimensional vector from
its noisy low frequency Fourier coefficients. We determine a sharp bound
on the min-max recovery error, and this is an immediate consequence of a
sharp bound on the smallest singular value of restricted Fourier
matrices. For the continuous model, we study the total variation
minimization method. We borrow ideas from Beurling in order to determine
general conditions for the recovery of singular measures, even those
that do not satisfy a minimum separation condition. This presentation
includes joint work with John Benedetto and Wenjing Liao.

Series: Algebra Seminar

A classical theorem in modern homotopy theory states that functors from finite pointed sets to spaces satisfying certain conditions model infinite loop spaces (Segal 1974). This theorem offers a recognition principle for infinite loop spaces. An analogous theorem for Morel-Voevodsky's motivic homotopy theory has been sought for since its inception. In joint work with Marc Hoyois, Adeel Khan, Vladimir Sosnilo and Maria Yakerson, we provide such a theorem. The category of finite pointed sets is replaced by a category where the objects are smooth schemes and the maps are spans whose "left legs" are finite syntomic maps equipped with a K-theoretic trivialization of its contangent complex. I will explain what this means, how it is not so different from finite pointed sets and why it was a natural guess. In particular, I will explain some of the requisite algebraic geometry.Time permitting, I will also provide 1) an explicit model for the motivic sphere spectrum as a torsor over a Hilbert scheme and,2) a model for all motivic Eilenberg-Maclane spaces as simplicial ind-smooth schemes.

Series: Geometry Topology Seminar

I'll introduce you to one of my favorite knotted objects: fibered,
homotopy-ribbon disk-knots. After giving a thorough overview of these
objects, I'll discuss joint work with Kyle Larson that brings some new
techniques to bear on their study. Then, I'll
present new work with Alex Zupan that introduces connections with Dehn
surgery and trisections. I'll finish by presenting a classification
result for fibered, homotopy-ribbon disk-knots bounded by square knots.

Series: PDE Seminar

The magnetohydrodynamic (MHD) equations govern the motion of electrically conducting fluids such as plasmas, liquid metals, and electrolytes. They consist of a coupled system of the Navier-Stokes equations of fluid dynamics and Maxwell's equations of electromagnetism. Besides their wide physical applicability, the MHD equations are also of great interest in mathematics. They share many similar features with the Navier-Stokes and the Euler equations. In the last few years there have been substantial developments on the global regularity problem concerning the magnetohydrodynamic (MHD) equations, especially when there is only partial or fractional dissipation. The talk presents recent results on the global well-posedness problem for the MHD equations with various partial or fractional dissipation.

Series: Research Horizons Seminar

Series: Analysis Seminar

We introduce a class of operators on abstract measurable spaces, which unifies variety of operators in Harmonic Analysis. We prove that such operators can be dominated by simple sparse operators. Those domination theorems imply some new estimations for Calderón-Zygmund operators, martingale transforms and Carleson operators.

Wednesday, October 4, 2017 - 13:55 ,
Location: Skiles 006 ,
Libby Taylor ,
Georgia Tech ,
Organizer: Jennifer Hom

Let K be a tame knot in S^3. Then the Alexander polynomial is knot invariant, which consists of a Laurent polynomial arising from the infinite cyclic cover of the knot complement. We will discuss the construction of the Alexander polynomial and, more generally, the Alexander invariant from a Seifert form on the knot. In addition, we will see some connections between the Alexander polynomial and other knot invariants, such as the genus and crossing number.

Series: School of Mathematics Colloquium

The study of nonconventional sums $S_{N}=\sum_{n=1}^{N}F(X(n),X(2n),\dots,X(\ell n))$, where $X(n)=g \circ T^n$ for a measure preserving transformation $T$, has a 40 years history after Furstenberg showed that they are related to the ergodic theory proof of Szemeredi's theorem about arithmetic progressions in the sets of integers of positive density. Recently, it turned out that various limit theorems of probabilty theory can be successfully studied for sums $S_{N}$ when $X(n), n=1,2,\dots$ are weakly dependent random variables. I will talk about a more general situation of nonconventional arrays of the form $S_{N}=\sum_{n=1}^{N}F(X(p_{1}n+q_{1}N),X(p_{2}n+q_{2}N),\dots,X(p_{\ell}n+q_{\ell}N))$ and how this is related to an extended version of Szemeredi's theorem. I'll discuss also ergodic and limit theorems for such and more general nonconventional arrays.

Series: School of Mathematics Colloquium

Series: Graph Theory Seminar

Let G be a graph containing 5 different vertices a0, a1, a2, b1
and b2. We say that (G, a0, a1, a2, b1, b2) is feasible if G contains
disjoint connected subgraphs G1, G2, such that {a0, a1, a2}⊆V(G1) and
{b1, b2}⊆V(G2). In this talk, we will describe
the structure of G when (G, a0, a1, a2, b1, b2) is infeasible, using
frames and connectors. Joint work with Changong Li, Robin Thomas, and
Xingxing Yu.

Series: Stochastics Seminar

The study of graph-partition problems such as Maxcut, max-bisection and
min-bisection have a long and rich history in combinatorics and theoretical
computer science. A recent line of work studies these problems on sparse random
graphs, via a connection with mean field spin glasses. In this talk, we will look
at this general direction, and derive sharp comparison inequalities between cut-sizes on sparse Erdös-Rényi and random regular graphs.
Based on joint work with Aukosh Jagannath.

Series: ACO Student Seminar

In a self-organizing particle system, an abstraction of programmable
matter, simple computational elements called particles with limited
memory and communication self-organize to solve system-wide problems of
movement, coordination, and configuration.
In this paper, we consider stochastic, distributed, local, asynchronous
algorithms for 'shortcut bridging', in which particles self-assemble
bridges over gaps that simultaneously balance minimizing the length and
cost of the bridge. Army ants of the genus Eticon
have been observed exhibiting a similar behavior in their foraging
trails, dynamically adjusting their bridges to satisfy an efficiency
tradeoff using local interactions. Using techniques from Markov chain
analysis, we rigorously analyze our algorithm, show
it achieves a near-optimal balance between the competing factors of path
length and bridge cost, and prove that it exhibits a dependence on the
angle of the gap being 'shortcut' similar to that of the ant bridges. We
also present simulation results that qualitatively
compare our algorithm with the army ant bridging behavior. Our work
presents a plausible explanation of how convergence to globally optimal
configurations can be achieved via local interactions by simple
organisms (e.g., ants) with some limited computational
power and access to random bits. The proposed algorithm demonstrates the
robustness of the stochastic approach to algorithms for programmable
matter, as it is a surprisingly simple extension of a stochastic
algorithm for compression.
This is joint work between myself/my professor Andrea Richa at ASU and Sarah Cannon and Prof. Dana Randall here at GaTech.

Series: Combinatorics Seminar

Given a (fixed) graph H, let X be the number of copies of H in the random binomial graph G(n,p). In this talk we recall the results on the asymptotic behaviour of X, as the number n of vertices grows and pis allowed to depend on. In particular we will focus on the problem of estimating probability that X is significantly larger than its expectation, which earned the name of the 'infamous upper tail'.

Friday, October 6, 2017 - 15:00 ,
Location: Skiles 154 ,
Sergio Mayorga ,
Georgia Tech ,
Organizer: Jiaqi Yang

We will look at a system of hamiltonian equations on the torus, with an
initial condition in momentum and a terminal condition in position, that
arises in mean field game theory. Existence of and uniqueness of
solutions will be shown, and a few remarks will be made in regard to its
connection to the minimization problem of a cost functional. This is the second part of lasrt week's talk.

Friday, October 6, 2017 - 15:00 ,
Location: Skiles 154 ,
Prof. Rafael de la Llave ,
School of Mathematics, Georgia Tech ,
Organizer: Jiaqi Yang

We will present an introduction to the results of S. Aubry and J. Mather who used variational methods to prove the existence of quasi-periodic orbits in twist mappings and in some models appearing in solid state Physics.