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Friday, April 20, 2018 - 15:05 ,
Location: Skiles 271 ,
Prof. Rafael de la Llave ,
GT Math ,
Organizer: Jiaqi Yang

A well known paper of H. Federer on Flat chains contains a remarkable example attributed to F. Almgren. We intend to give a geometric exposition of the example and explain its relevance in the global theory of geodesic flows and some global problems such as homogenization in quasi-periodic media. This is part of an expository paper with X. Su.

Series: ACO Student Seminar

Consider the problem of selling items to a unit-demand buyer. Most work
on maximizing seller revenue considers either a setting that is single
dimensional, such as where the items are identical, or
multi-dimensional, where the items are heterogeneous. With respect to
revenue-optimal mechanisms, these settings sit at extreme ends of a
spectrum: from simple and fully characterized (single-dimensional) to
complex and nebulous (multi-dimensional).
In this paper, we identify a setting that sits in between these
extremes. We consider a seller who has three services {A,B,C} for sale
to a single buyer with a value v and an interest G from {A,B,C}, and
there is a known partial ordering over the services. For example,
suppose the seller is selling {internet}, {internet, phone}, and
{internet, cable tv}. A buyer with interest {internet} would be
satisfied by receiving phone or cable tv in addition, but a customer
whose interest is {internet, phone} cannot be satisfied by any other
option. Thus this corresponds to a partial-ordering where {internet}
> {internet, phone} and {internet} > {internet, cable tv}, but
{internet, phone} and {internet, cable tv} are not comparable.
We show formally that partially-ordered items lie in a space of their
own, in between identical and heterogeneous items: there exist
distributions over (value, interest) pairs for three partially-ordered
items such that the menu complexity of the optimal mechanism is
unbounded, yet for all distributions there exists an optimal mechanism
of finite menu complexity. So this setting is vastly more complex than
identical items (where the menu complexity is one), or even
“totally-ordered” items as in the FedEx Problem [FGKK16] (where the menu
complexity is at most seven, for three items), yet drastically more
structured than heterogeneous items (where the menu complexity can be
uncountable [DDT15]). We achieve this result by proving a
characterization of the class of best duals and by giving a primal
recovery algorithm which obtains the optimal mechanism. In addition, we
(1) extend our lower-bound to the Multi-Unit Pricing setting, (2) give a
tighter and deterministic characterization of the optimal mechanism
when the buyer’s distribution satisfies the declining marginal revenue
condition, and (3) prove a master theorem that allows us to reason about
duals instead of distributions.
Joint work with Nikhil Devanur, Raghuvansh Saxena, Ariel Schvartzman, and Matt Weinberg.

Friday, April 20, 2018 - 10:00 ,
Location: Skiles 006 ,
Jose Acevedo ,
Georgia Tech ,
Organizer: Kisun Lee

In this talk we show how to obtain some (sometimes sharp) inequalities between subgraph densities which are valid asymptotically on any sequence of finite simple graphs with an increasing number of vertices. In order to do this we codify a simple graph with its edge monomial and establish a nice graphical notation that will allow us to play around with these densities.

Series: Stochastics Seminar

We shall prove that a certain stochastic ordering defined in terms of
convex symmetric sets is inherited by sums of independent symmetric
random vectors. Joint work with W. Bednorz.

Series: Graph Theory Seminar

Györi and Lovasz independently proved that a k-connected graph can be partitioned into k subgraphs, with each subgraph connected, containing a prescribed vertex, and with a prescribed
vertex count. Lovasz used topological methods, while Györi found a purely graph theoretical approach. Chen et al. later generalized the topological proof to graphs with weighted
vertices, where the subgraphs have prescribed weight sum rather than vertex count. The weighted result was recently proven using Györi's approach by Chandran et al. We will use the
Györi approach to generalize the weighted result slightly further. Joint work with Robin Thomas.

Series: School of Mathematics Colloquium

This talk is about the structure theory of measure-preserving systems: transformations of a finite measure space that preserve the measure. Many important examples arise from stationary processes in probability, and simplest among these are the i.i.d. processes. In ergodic theory, i.i.d. processes are called Bernoulli shifts. Some of the main results of ergodic theory concern an invariant of systems called their entropy, which turns out to be intimately related to the existence of `structure preserving' maps from a general system to Bernoulli shifts. I will give an overview of this area and its history, ending with a recent advance in this direction. A measure-preserving system has the weak Pinsker property if it can be split, in a natural sense, into a direct product of a Bernoulli shift and a system of arbitrarily low entropy. The recent result is that all ergodic measure-preserving systems have this property. This talk will assume graduate-level real analysis and measure theory, and familiarity with the basic language of random variables. Past exposure to entropy, measure-theoretic probability or ergodic theory will be helpful, but not essential.

Wednesday, April 18, 2018 - 14:10 ,
Location: Skiles 006 ,
Sarah Davis ,
GaTech ,
Organizer: Anubhav Mukherjee

The theorem of Dehn-Nielsen-Baer says the extended mapping class group is isomorphic to the outer automorphism group of the fundamental group of a surface. This theorem is a beautiful example of the interconnection between purely topological and purely algebraic concepts. This talk will discuss the background of the theorem and give a sketch of the proof.

Series: Analysis Seminar

We discuss the probability that a continuous stationary Gaussian process on whose spectral measure vanishes in a neighborhood of the origin stays non-negative on an interval of long interval. Joint work with Naomi Feldheim, Ohad Feldheim, Fedor Nazarov, and Shahaf Nitzan

Series: PDE Seminar

Epitaxial growth is an important physical process for forming solid films or other nano-structures. It occurs as atoms, deposited from above, adsorb and diffuse on a crystal surface. Modeling the rates that atoms hop and break bonds leads in the continuum limit to degenerate 4th-order PDE that involve exponential nonlinearity and the p-Laplacian with p=1, for example. We discuss a number of analytical results for such models, some of which involve subgradient dynamics for Radon measure solutions.

Series: Geometry Topology Seminar

Augmentations and exact Lagrangian fillings are closely related. However, not all the augmentations of a Legendrian knot come from embedded exact Lagrangian fillings. In this talk, we show that all the augmentations come from possibly immersed exact Lagrangian fillings. In particular, let ∑ be an immersed exact Lagrangian filling of a Legendrian knot in $J^1(M)$ and suppose it can be lifted to an embedded Legendrian L in J^1(R \times M). For any augmentation of L, we associate an induced augmentation of the Legendrian knot, whose homotopy class only depends on the compactly supported Legendrian isotopy type of L and the homotopy class of its augmentation of L. This is a joint work with Dan Rutherford.