## Seminars and Colloquia by Series

### Thesis Defense: The Maxwell-Pauli Equations

Series
PDE Seminar
Time
Tuesday, March 10, 2020 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Thomas KiefferGeorgia Tech

Energetic stability of matter in quantum mechanics, which refers to the question of whether the ground state energy of a

many-body quantum mechanical system is finite, has long been a deep question of mathematical physics. For a system of many
non-relativistic electrons interacting with many nuclei in the absence of electromagnetic fields this question traces back
to the seminal works of Tosio Kato in 1951 and Freeman Dyson and Andrew Lenard in 1967/1968. In particular, Dyson and Lenard
showed the ground state energy of the many-body Schrödinger Hamiltonian is bounded below by a constant times the total particle
number, regardless of the size of the nuclear charges. This situation changes dramatically when electromagnetic fields and spin
interactions are present in the problem. Even for a single electron with spin interacting with a single nucleus of charge
$Z > 0$ in an external magnetic field, Jurg Fröhlich, Elliot Lieb, and Michael Loss in 1986 showed that there is no ground state
energy if $Z > Z_c$ and the ground state energy exists if $Z < Z_c$.

Another notion of stability in quantum mechanics is that of dynamic stability. Dynamic stability refers to the question of global
well-posedness for a system of partial differential equations that models the dynamics of many electrons coupled to their
self-generated electromagnetic field and interacting with many nuclei. The central motivating question of our PhD thesis is
whether energetic stability has any influence on the global well-posedness of the corresponding dynamical equations. In this regard,
we study the quantum mechanical many-body problem of $N$ non-relativistic electrons with spin interacting with their self-generated classical electromagnetic field and $K$ static nuclei. We model the dynamics of the electrons and their self-generated
electromagnetic field using the so-called many-body Maxwell-Pauli equations. The main result presented is the construction
time global, finite-energy, weak solutions to the many-body Maxwell-Pauli equations under the assumption that the fine structure
constant $\alpha$ and the nuclear charges are sufficiently small to ensure energetic stability of this system. If time permits, we
will discuss several open problems that remain.

### The Maxwell-Pauli Equations

Series
Dissertation Defense
Time
Tuesday, March 10, 2020 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Forrest KiefferGeorgia Institute of Technology

Energetic stability of matter in quantum mechanics, which refers to the ques-
tion of whether the ground state energy of a many-body quantum mechanical
system is finite, has long been a deep question of mathematical physics. For a
system of many non-relativistic electrons interacting with many nuclei in the
absence of electromagnetic fields this question traces back to the seminal work
of Tosio Kato in 1951 and Freeman Dyson and Andrew Lenard in 1967/1968.
In particular, Dyson and Lenard showed the ground state energy of the many-
body Schrödinger Hamiltonian is bounded below by a constant times the total
particle number, regardless of the size of the nuclear charges. This says such a
system is energetically stable (of the second kind). This situation changes dra-
matically when electromagnetic fields and spin interactions are present in the
problem. Even for a single electron with spin interacting with a single nucleus
of charge $Z > 0$ in an external magnetic field, Jurg Fröhlich, Elliot Lieb, and
Michael Loss in 1986 showed that there is no ground state energy if $Z$ exceeds
a critical charge $Z_c$ and the ground state energy exists if $Z < Z_c$ . In other
words, if the nuclear charge is too large, the one-electron atom is energetically
unstable.

Another notion of stability in quantum mechanics is that of dynamic stabil-
ity, which refers to the question of global well-posedness for a system of partial
differential equations that models the dynamics of many electrons coupled to
their self-generated electromagnetic field and interacting with many nuclei. The
central motivating question of our PhD thesis is whether energetic stability has
any influence over dynamic stability. Concerning this question, we study the
quantum mechanical many-body problem of $N \geq 1$ non-relativistic electrons with
spin interacting with their self-generated classical electromagnetic field and $K \geq 0$
static nuclei. We model the dynamics of the electrons and their self-generated
electromagnetic field using the so-called many-body Maxwell-Pauli equations.
The main result presented is the construction time global, finite-energy, weak
solutions to the many-body Maxwell-Pauli equations under the assumption that
the fine structure constant $\alpha$ and the nuclear charges are sufficiently small to
ensure energetic stability of this system. This result represents an initial step
towards understanding the relationship between energetic stability and dynamic
stability. If time permits, we will discuss several open problems that remain.

Committee members: Prof. Michael Loss (Advisor, School of Mathematics,
Georgia Tech), Prof. Brian Kennedy (School of Physics, Georgia Tech), Prof.
Evans Harrell (School of Mathematics, Georgia Tech), Prof. Federico Bonetto
(School of Mathematics, Georgia Tech), Prof. Chongchun Zeng (School of Math-
ematics, Georgia Tech).

### Generating functions for induced characters of the hyperoctahedral group

Series
Algebra Seminar
Time
Monday, March 9, 2020 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Mark SkanderaLehigh University

Merris and Watkins interpreted results of Littlewood to give generating functions for symmetric group characters induced from one-dimensional characters of Young subgroups.  Beginning with an n by n matrix X of formal variables, one obtains induced sign and trivial characters by expanding sums of products of certain determinants and permanents, respectively. We will look at a new analogous result which holds for hyperoctahedral group characters induced from the four one-dimensional characters of its Young subgroups.  This requires a 2n by 2n matrix of formal variables and four combinations of determinants and permanents.  This is joint work with Jongwon Kim.

### Satellite operations and knot genera

Series
Geometry Topology Seminar
Time
Monday, March 9, 2020 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Allison MillerRice University

The satellite construction, which associates to a pattern knot P in a solid torus and a companion knot K in the 3-sphere the so-called satellite knot P(K), features prominently in knot theory and low-dimensional topology.  Besides the intuition that P(K) is “more complicated” than either P or K, one can attempt to quantify how the complexity of a knot changes under the satellite operation. In this talk, I’ll discuss how several notions of complexity based on the minimal genus of an embedded surface change under satelliting. In the case of the classical Seifert genus of a knot, Schubert gives an exact formula. In the 4-dimensional context the situation is more complicated, and depends on whether we work in the smooth or topological category: the smooth category is sometimes asymptotically similar to the classical setting, but our main results show that the topological category is much weirder.  This talk is based on joint work with Peter Feller and Juanita Pinzón-Caicedo.

### Wobbling of pedestrian bridges

Series
Applied and Computational Mathematics Seminar
Time
Monday, March 9, 2020 - 13:55 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Guillermo GoldszteinGeorgia Tech

On June 10, 2000, the Millennium Bridge in London opened to the public. As people crossed the bridge, it wobbled. The sway of the bridge was large enough that prompted many on the bridge to hold on to the rails. Three days later, the bridge closed. It reopened only after modifications to prevent the wobbling were made, eighteen months later. We develop and study a model motivated by this event

### Small torsion generating sets for mapping class groups

Series
Dissertation Defense
Time
Monday, March 9, 2020 - 11:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Justin LanierGeorgia Tech

A surface of genus $g$ has many symmetries. These form the surface’s mapping class group $Mod(S_g)$, which is finitely generated. The most commonly used generating sets for $Mod(S_g)$ are comprised of infinite order elements called Dehn twists; however, a number of authors have shown that torsion generating sets are also possible. For example, Brendle and Farb showed that $Mod(S_g)$ is generated by six involutions for $g \geq 3$. We will discuss our extension of these results to elements of arbitrary order: for $k > 5$ and $g$ sufficiently large, $Mod(S_g)$ is generated by three elements of order $k$. Generalizing this idea, in joint work with Margalit we showed that for $g \geq 3$ every nontrivial periodic element that is not a hyperelliptic involution normally generates $Mod(S_g)$. This result raises a question: does there exist an $N$, independent of $g$, so that if $f$ is a periodic normal generator of $Mod(S_g)$, then $Mod(S_g)$ is generated by $N$ conjugates of $f$? We show that in general there does not exist such an $N$, but that there does exist such a universal bound for the class of non-involution normal generators.

### Graph fractal dimension and structure of fractal networks: a combinatorial perspective

Series
Combinatorics Seminar
Time
Friday, March 6, 2020 - 15:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Pavel SkumsGeorgia State University

We study self-similar and fractal networks from the combinatorial perspective. We establish analogues of topological (Lebesgue) and fractal (Hausdorff) dimensions for graphs and demonstrate that they are naturally related to known graph-theoretical characteristics: rank dimension and product (or Prague or Nešetřil-Rödl) dimension. Our approach reveals how self-similarity and fractality of a network are defined by a pattern of overlaps between densely connected network communities. It allows us to identify fractal graphs, explore the relations between graph fractality, graph colorings and graph Kolmogorov complexity, and analyze the fractality of several classes of graphs and network models, as well as of a number of real-life networks. We demonstrate the application of our framework to evolutionary studies by revealing the growth of self-organization of heterogeneous viral populations over the course of their intra-host evolution, thus suggesting mechanisms of their gradual adaptation to the host's environment. As far as the authors know, the proposed approach is the first theoretical framework for study of network fractality within the combinatorial paradigm. The obtained results lay a foundation for studying fractal properties of complex networks using combinatorial methods and algorithms.

Based on joint work with Leonid Bunimovich

### Strong self concordance and sampling

Series
ACO Student Seminar
Time
Friday, March 6, 2020 - 13:05 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker

Motivated by the Dikin walk, we develop aspects of an interior-point

theory for sampling in high dimensions. Specifically, we introduce symmetric

and strong self-concordance. These properties imply that the corresponding

Dikin walk mixes in $\tilde{O}(n\bar{\nu})$ steps from a warm start

in a convex body in $\mathbb{R}^{n}$ using a strongly self-concordant barrier

with symmetric self-concordance parameter $\bar{\nu}$. For many natural

barriers, $\bar{\nu}$ is roughly bounded by $\nu$, the standard

self-concordance parameter. We show that this property and strong

self-concordance hold for the Lee-Sidford barrier. As a consequence,

we obtain the first walk to mix in $\tilde{O}(n^{2})$ steps for an

arbitrary polytope in $\mathbb{R}^{n}$. Strong self-concordance for other

barriers leads to an interesting (and unexpected) connection ---

for the universal and entropic barriers, it is implied by the KLS

conjecture.

### Resonant tori of arbitrary codimension for quasi-periodically forced systems

Series
Math Physics Seminar
Time
Thursday, March 5, 2020 - 16:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Guido GentileUniversita&#039; di Roma 3

Consider a system of rotators subject to a small quasi-periodic forcing which (1) is analytic, (2) satisfies a time-reversibility property, and (3) has a Bryuno frequency vector. Without imposing any non-degeneracy condition, we prove that there exists at least one quasi-periodic solution with the same frequency vector as the forcing. The result can be interpreted as a theorem of persistence of lower-dimensional tori of arbitrary codimension in degenerate cases. This is a joint work with Livia Corsi.