## Spring 2021

Date | Speaker | Affiliation | Title |
---|---|---|---|

Feb 3 | David Garcı́a-Zelada | Aix-Marseille U | A large deviation principle for empirical measures |

Feb 10 | Arno Kuijlaars | KU Leuven | The spherical ensemble with external sources |

Feb 17 | Alex Iosevich | U of Rochester | Finite point configurations and frame theory |

Feb 24 | Kasso Okoudjou | Tufts U | Completeness of Weyl-Heisenberg POVMs |

Mar 3 | Mircea Petrache | PUC Chile | Sharp isoperimetric inequality, discrete PDEs and semidiscrete optimal transport |

Mar 10 | Yeli Niu | U of Alberta | Discretization of integrals on compact metric measure spaces |

Mar 17 | Xuemei Chen | UNC Wilmington | Frame Design Using Projective Riesz Energy |

Mar 24 | Ruiwen Shu | U of Maryland | Dynamics of Particles on a Curve with Pairwise Hyper-singular Repulsion |

Mar 31 | Oleg Musin | U of Texas Rio Grande Valley | Majorization, discrete energy on spheres and f-designs |

Apr 7 | Woden Kusner | U of Georgia | Measuring chirality with the wind |

**David Garcı́a-Zelada**(Aix-Marseille U)

*A large deviation principle for empirical measures*

Abstract

The main object of this talk will be a model of $n$ interacting particles at equilibrium. I will describe its macroscopic behavior as $n$ grows to infinity by showing a Laplace principle or, equivalently, a large deviation principle. This implies, in some cases, an almost sure convergence to a deterministic probability measure. Among the main motivating examples we may find Coulomb gases on Riemannian manifolds, the eigenvalue distribution of Gaussian random matrices and the roots of Gaussian random polynomials. This talk is based on arXiv:1703.02680.

Slides: [pdf]

Video: [mp4]

Paper: [arXiv]

**Arno Kuijlaars**(KU Leuven)

*The spherical ensemble with external sources*

Abstract

The talk is based on joint work with Juan Criado del Rey.

We study a model of a large number of points on the unit sphere under the influence of a finite number of fixed repelling charges. In the large n limit the points fill a region that is known as the droplet. For small external charges the droplet is the complement of the union of a number of spherical caps, one around each of the external charges. When the external charges grow, the spherical caps will start to overlap and the droplet ondergoes a non-trivial deformation.

We explicitly describe the transition for the case of equal external charges that are symmetrically located around the north pole. In our approach we first identify a motherbody that, due to the symmetry in the problem, will be located on a number of meridians connecting the north and south poles. After projecting onto the complex plane, and undoing the symmetry, we characterize the motherbody by means of the solution of a vector equilibrium problem from logarithmic potential theory.

We study a model of a large number of points on the unit sphere under the influence of a finite number of fixed repelling charges. In the large n limit the points fill a region that is known as the droplet. For small external charges the droplet is the complement of the union of a number of spherical caps, one around each of the external charges. When the external charges grow, the spherical caps will start to overlap and the droplet ondergoes a non-trivial deformation.

We explicitly describe the transition for the case of equal external charges that are symmetrically located around the north pole. In our approach we first identify a motherbody that, due to the symmetry in the problem, will be located on a number of meridians connecting the north and south poles. After projecting onto the complex plane, and undoing the symmetry, we characterize the motherbody by means of the solution of a vector equilibrium problem from logarithmic potential theory.

Slides: [pdf]

Video: [mp4]

Paper: [arXiv]

**Alex Iosevich**(U of Rochester)

*Finite point configurations and frame theory*

Abstract

We are going to discuss some recent and not so recent applications
of analytic and combinatorial results on finite point configurations to
problems of existence of exponential and Gabor frames and bases.

Slides: [pdf]

Video: [mp4]

**Kasso Okoudjou**(Tufts U)

*Completeness of Weyl-Heisenberg POVMs*

Abstract

The finite Gabor (or Weyl-Heisenberg) system generated by a unit-norm vector
$g\in \mathbb{C}^d$ is the set of vectors $$\big\{g_{k,\ell}=e^{2\pi i
k\cdot}g(\cdot - \ell)\big\}_{k, \ell =0}^{d-1}.$$ It is know that every such
system forms a finite unit norm tight frame (FUNTF) for $\mathbb{C}^d$, i.e., $$d^3
\|x\|^2=\sum_{k, \ell=0}^{d-1}|\langle x, g_{k,\ell}\rangle |^2\quad \forall
\, x\in \mathbb{C}^d.$$ Furthermore, the Zauner conjecture asserts that for
each $d\geq 2$, there exist unit-norm vectors $g \in \mathbb{C}^d$ such that
this FUNTF is equiangular, that is, $|\langle g, g_{k, \ell}\rangle |^2=
\tfrac{1}{d+1}.$
Assuming the existence of a unit-vector $g$ that positively answers Zauner's
conjecture, one can show that the set of rank-one matrices
$$\big\{\pi_{k,\ell}=\langle \cdot, g_{k,\ell}\rangle g_{k, \ell}\big\}_{k
\ell=0}^{d-1}$$ is complete in the space of $d\times d$ matrices.
Consequently, $\big\{\pi_{k,\ell}\big\}_{k \ell=0}^{d-1}$ forms a symmetric
informationally complete positive operator-valued measure (SIC-POVM).
In fact, it is known that given a unit-norm vector $g\in \mathbb{C}^d$, the
POVM $\big\{\pi_{k,\ell}\big\}_{k \ell=0}^{d-1}$ is informationally complete
(IC) if and only if $\langle g, g_{k, \ell} \rangle \neq 0$ for all $(k
,\ell)\neq (0,0)$.
In this talk, we give a different proof of the characterization of the
IC-POVMs. We then focus on investigating non-informationally complete POVMs.
We will present some preliminary results pertaining to the dimensions of the
linear spaces spanned by these rank-one matrices. (This talk is based on
on-going joint work with S. Kang and A. Goldberger.)

Video: [mp4]

**Mircea Petrache**(PUC Chile)

*Sharp isoperimetric inequality, discrete PDEs and semidiscrete optimal transport*

Abstract

Consider the following basic model of finite crystal cluster
formation: in a periodic graph G with vertices in R^d (representing
possible molecular bonds) a subset (of atoms) must be chosen, so that
the total number of bonds between a point in X and one outside X is
minimized. These bonds form the edge-perimeter of X, denoted \partial
X.
If the graph is periodic and locally finite, any X satisfies an
inequality of the form |X|^{d-1} \leq C |partial X|^d, where the
optimal C depends on the graph. How can we determine the structure of
sets X realizing equality in the above, based on the geometry and of
G?
If we take the continuum limit of G, then the classical Wulff shape
theory describes optimal limit shapes, and at least two proofs of
isoperimetric inequality apply, one based on PDE and calibration
ideas, and the other based on Optimal Transport ideas. We focus on
using the heuristic coming from the continuum analogue, to answer the
above question in some cases, in the discrete case. This approach
highlights the tight connection between discrete PDEs and semidiscrete
Optimal Transport, and a link to the Minkowski theorem for convex
polyhedra.

Paper: [arXiv1] [arXiv2]

Slides: [pdf]

Video: [mp4]

**9:30am CST/10:30am EST/4:30pm CET**

**Yeli Niu**(U of Alberta)

*Discretization of integrals on compact metric measure spaces*

Abstract

Let $\mu$ be a Borel probability measure on a compact path-connected
metric space $(X, \rho)$ for which there exist constants $c,\beta\ge 1$ such that
$\mu(B) \ge c r^{\beta}$ for every open ball $B\subset X$ of radius $r>0$. For a
class of Lipschitz functions $\Phi:[0,\infty)\to\mathbb R$ that are piecewise within
a finite-dimensional subspace of continuous functions, we prove under certain
mild conditions on the metric $\rho$ and the measure $\mu$ that for each
positive integer $N\ge 2$, and each $g\in L^\infty(X, d\mu)$ with
$\|g\|_\infty=1$, there exist points $y_1, \ldots, y_{ N }\in X$ and real
numbers $\lambda_1, \ldots, \lambda_{ N }$ such that for any $x\in X$,
\begin{align*} & \left| \int_X \Phi (\rho (x, y)) g(y) \,\text{d} \mu (y) -
\sum_{j = 1}^{ N } \lambda_j \Phi (\rho (x, y_j)) \right| \leqslant C N^{-
\frac{1}{2} - \frac{3}{2\beta}} \sqrt{\log N}, \end{align*} where the constant
$C>0$ is independent of $N$ and $g$. In the case when $X$ is the unit sphere
$\mathbb{S}^{d}$ of $\mathbb R^{d+1}$ with the ususal geodesic distance, we also prove that the
constant $C$ here is independent of the dimension $d$. Our estimates are
better than those obtained from the standard Monte Carlo methods, which
typically yield a weaker upper bound $N^{-\frac 12}\sqrt{\log N}$.

Paper: [arXiv]

Slides: [pdf]

Video: [mp4]

**Xuemei Chen**(UNC Wilmington)

*Frame Design Using Projective Riesz Energy*

Abstract

Tight and well-separated frames are desirable in many signal
processing applications. We introduce a projective Riesz kernel for the
unit sphere and investigate properties of N-point energy minimizing
configurations for such a kernel. We show that these minimizing
configurations, for N sufficiently large, form frames that are
well-separated (have low coherence) and are nearly tight. We will also
show some numerical experiments. This is joint work with Doug Hardin and
Ed Saff.

Paper: [arXiv]

Slides: [pdf]

Video: [mp4]

**Ruiwen Shu**(U of Maryland)

*Dynamics of Particles on a Curve with Pairwise Hyper-singular Repulsion*

Abstract

We investigate the large time behavior of $N$ particles restricted
to a smooth closed curve in $\mathbb{R}^d$ and subject to a gradient flow with
respect to Euclidean hyper-singular repulsive Riesz $s$-energy with $s>1$. We
show that regardless of their initial positions, for all $N$ and time $t$
large, their normalized Riesz $s$-energy will be close to the $N$-point
minimal possible energy. Furthermore, the distribution of such particles will
be close to uniform with respect to arclength measure along the curve.

Paper: [arXiv]

Slides: [pdf]

Video: [mp4]

**Oleg Musin**(U of Texas Rio Grande Valley)

*Majorization, discrete energy on spheres and f-designs*

Abstract

We consider the majorization (Karamata) inequality and minimums of the
majorization (M-sets) for f-energy potentials of m-point configurations in a
sphere. We discuss the optimality of regular simplexes, describe M-sets with
a small number of points, define spherical f-designs and study their
properties. Then we consider relations between the notions of f-designs and
M-sets, $\tau$-designs, and two-distance sets

Paper: [arXiv]

Slides: [pdf]

Video: [mp4]

**Woden Kusner**(U of Georgia)

*Measuring chirality with the wind*

Abstract

The question of measuring "handedness" is of some significance in
both mathematics and in the real world. Propellors and screws, proteins and
DNA, in fact *almost everything* is chiral. Can we quantify chirality? Or
can we perhaps answer the question:

"Are your shoes more left-or-right handed than a potato?"

We can begin with the hydrodynamic principle that chiral objects rotate when placed in a collimated flow (or wind). This intuition naturally leads to a trace-free tensorial chirality measure for space curves and surfaces, with a clear physical interpretation measuring twist. As a consequence, the "average handedness" of an object with respect to this measure will always be 0. This also strongly suggests that a posited construction of Lord Kelvin--the isotropic helicoid--can not exist.

joint with Giovanni Dietler, Rob Kusner, Eric Rawdon and Piotr Szymczak

"Are your shoes more left-or-right handed than a potato?"

We can begin with the hydrodynamic principle that chiral objects rotate when placed in a collimated flow (or wind). This intuition naturally leads to a trace-free tensorial chirality measure for space curves and surfaces, with a clear physical interpretation measuring twist. As a consequence, the "average handedness" of an object with respect to this measure will always be 0. This also strongly suggests that a posited construction of Lord Kelvin--the isotropic helicoid--can not exist.

joint with Giovanni Dietler, Rob Kusner, Eric Rawdon and Piotr Szymczak

Paper: [arXiv]

Slides: [pdf]

Video: [mp4]

**Peter Dragnev**(Purdue Fort Wayne)

*Bounds for Spherical Codes: The Levenshtein Framework Lifted*

Abstract

Based on the Delsarte-Yudin linear programming approach, we
extend Levenshtein’s framework to obtain lower bounds for the minimum henergy
of spherical codes of prescribed dimension and cardinality, and upper
bounds on the maximal cardinality of spherical codes of prescribed dimension
and minimum separation. These bounds are universal in the sense that
they hold for a large class of potentials h and in the sense of Levenshtein.
Moreover, codes attaining the bounds are universally optimal in the sense of
Cohn-Kumar. Referring to Levenshtein bounds and the energy bounds of the
authors as “first level”, our results can be considered as “next level”
universal
bounds as they have the same general nature and imply necessary and sufficient
conditions for their local and global optimality. For this purpose, we
introduce the notion of Universal Lower Bound space (ULB-space), a space
that satisfies certain quadrature and interpolation properties. While there
are
numerous cases for which our method applies, we will emphasize the model
examples of 24 points (24-cell) and 120 points (600-cell) on $\mathbb{S}^3$.
In particular,
we provide a new proof that the 600-cell is universally optimal, and in so
doing,
we derive optimality of the 600-cell on a class larger than the absolutely
monotone potentials considered by Cohn-Kumar.

Slides: [pdf]

Video: [mp4]

## Fall 2020

Date | Speaker | Affiliation | Title |
---|---|---|---|

Sep 16 | Dmitriy Bilyk | U of Minnesota | Stolarsky principle: generalizations, extensions, and applications |

Sep 23 | Alexander Reznikov | Florida State | Minimal discrete energy on fractals |

Sep 30 | Stefan Steinerberger | U of Washington | Optimal Transport and Point Distributions on the Torus |

Oct 2 | Stefan Steinerberger | U of Washington | Optimal Transport and Point Distributions on Manifolds |

Oct 7 | Oleksandr Vlasiuk | Florida State | Asymptotic properties of short-range interaction functionals |

Oct 14 | Peter Grabner | TU Graz | Fourier-Eigenfunctions and Modular Forms |

Oct 21 | Michelle Mastrianni | U of Minnesota | Bounds for Star-Discrepancy with Dependence on the Dimension |

Oct 28 | Carlos Beltrán | U of Cantabria | Smale’s motivation in describing the 7th problem of his list |

Nov 4 | Adrian Ebert | RICAM | Construction of (polynomial) lattice rules by smoothness-independent component-by-component digit-by-digit constructions |

Nov 11 | Alexey Glazyrin | U of Texas Rio Grande Valley | Mapping to the space of spherical harmonics |

Dec 2 | Alexander Barg | U of Maryland | Stolarsky's invariance principle for the Hamming space |

Dec 9 | Paul Leopardi | Australian National U | Diameter bounded equal measure partitions of Ahlfors regular metric measure spaces |

Dec 16 | Laurent Bétermin | U of Vienna | Theta functions, ionic crystal energies and optimal lattices |

**Dmitriy Bilyk**(U of Minnesota)

*Stolarsky principle: generalizations, extensions, and applications*

Abstract

In 1973 Kenneth Stolarsky proved a remarkable identity, which connected two
classical quantities, which measure the quality of point distributions on
the sphere: the $L^2$ spherical cap discrepancy and the pairwise sum of
Euclidean distances between points. This fact, which came to be known as
the Stolarsky Invariance Principle, established a certain duality between
problems of discrepancy theory on one hand, and distance geometry or energy
optimization on the other, and allowed one to transfer methods and results
of one field to the other. Since then numerous versions, extensions, and
generalizations of this principle have been found, leading to connections
between various notions of discrepancy and discrete energies in different
settings and to a number of applications to various problems of discrete
geometry. In this talk we shall survey known work on the Stolarsky
principle, as well as some related problems.

Slides: [pdf]

Video: [mp4]

**Alexander Reznikov**(Florida State)

*Minimal discrete energy on fractals*

Abstract

We will survey some old and new results on the existence of
asymptotic behavior of minimal discrete Riesz energy of many particles
located in a fractal set. Unlike in the case of a rectifiable set,
when the asymptotic behavior always exists, we will show that on a
large class of somewhat "balanced" fractals the energy (and
best-packing) does not have any asymptotic behavior.

Slides: [pdf]

Video: [mp4]

**Stefan Steinerberger**(U of Washington)

*Optimal Transport and Point Distributions on the Torus*

Abstract

There are lots of ways of measuring the regularity of a set
of points on the Torus. I'll introduce a fundamental notion from Optimal
Transport, the Wasserstein distance, as another such measure. It
corresponds quite literally over what distance one has to spread the
points to be evenly distributed, it has a natural physical intuition
(the notion itself was derived in Economics modeling transport) and is
naturally related to other notions such as discrepancy or Zinterhof's
diaphony. Classical Fourier Analysis allows us to bound this transport
distance via exponential sums which are well studied; this allows us to
revisit
many classical constructions and get transport bounds basically for free.
We'll finish by revisiting a classical problem from numerical integration
from this new angle. There will be many open problems throughout the talk.

Slides: [pdf]

Video: [mp4]

**Stefan Steinerberger**(U of Washington)

*Optimal Transport and Point Distributions on Manifolds*

Abstract

We'll go somewhat deeper into the connection between the
Wasserstein distance and notions from potential theory: in particular,
how the classical Green function can be used to derive bounds on
Wasserstein transport on general manifolds. On the sphere, our results
simplify and the Riesz energy appears in a nice form. We conclude with
a fundamental new idea: the Wasserstein Uncertainty Principle which
says that if it's terribly easy to buy milk wherever you are, then there
must be many supermarkets -- the precise form of this isoperimetric
principle is not known and, despite being purely geometric, it would
have immediate impact on some PDE problems.

Slides: (coming soon)

Video: [mp4]

**Oleksandr Vlasiuk**(Florida State)

*Asymptotic properties of short-range interaction functionals*

Abstract

Short-range interactions, such as the hypersingular Riesz energies, are known to be amenable to asymptotic analysis, which allows to obtain for them the distribution of minimizers and asymptotics of the minima. We extract the properties making such analysis possible into a standalone framework. This allows us to give a unified treatment of hypersingular Riesz energies and optimal quantizers. We further obtain new results about the scale-invariant nearest neighbor interactions, such as the $ k $-nearest neighbor truncated Riesz energy. The suggested approach has applications to common methods for generating distributions with prescribed density: Riesz energies, centroidal Voronoi tessellations, and popular meshing algorithms due to Persson-Strang and Shimada-Gossard. It naturally generalizes from 2-body to $k$-body interactions.

Based on joint work with Douglas Hardin and Ed Saff.

Based on joint work with Douglas Hardin and Ed Saff.

Slides: [pdf]

Video: [mp4]

**Peter Grabner**(TU Graz)

*Fourier-Eigenfunctions and Modular Forms*

Abstract

Eigenfunctions of the Fourier-transform play a major role in Viazovska's
proof of the best packing of the $E_8$ lattice in dimension 8 and the
subsequent determination of the Leech lattice as best packing
configuration dimension 24 by Cohn, Kumar, Miller, Radchenko, and
Viazovska. In joint work with A. Feigenbaum and D. Hardin we have shown
that the constructions as used for these results are unique; we could
shed more light on the underlying modular and quasimodular forms and
determine linear recurrence relations and differential equations
characterising these forms.

Slides: (coming soon)

Paper: [arXiv1] [arXiv2]

Video: [mp4]

**Michelle Mastrianni**(U of Minnesota)

*Bounds for Star-Discrepancy with Dependence on the Dimension*

Abstract

The question of how the star-discrepancy (with respect to corners) of an
n-point set in the d-dimensional unit cube depends on the dimension d was
studied in 2001 by Heinrich, Novak, Wasilkowski and Wozniakowski. They
established an upper bound that depends only polynomially on d/n. The proof
makes use of the fact that the set of corners in the d-dimensional unit
cube is a VC-class, and employs a result by Talagrand (1994) that uses a
partitioning scheme to study the tails of the supremum of a Gaussian
process under certain conditions that are always satisfied by VC-classes.
In 2011, Aistleitner produced a simpler proof of this upper bound using a
direct dyadic partitioning argument and explicitly computed the constant;
an improvement on the constant was given by Pasing and Weiss in 2018. The
best lower bound was achieved by Hinrichs (2003), who built upon the ideas
of using VC-inequalities to achieve a lower bound with polynomial behavior
in d/n as well. In this talk I will introduce the notion of VC dimension
and discuss how it is employed in the above proofs, and outline how the
direct partitioning argument for the upper bound uses the same underlying
ideas about where the bulk of the contribution to the tails arises.

Slides: [pdf]

Video: [mp4]

**3pm CET**(Paris, Berlin)

**Carlos Beltrán**(U of Cantabria)

*Smale’s motivation in describing the 7th problem of his list*

Abstract

In 1993, Mike Shub and Steve Smale posed a question that would be later
included in Smale’s list as 7th problem. Although this last problem has
became so famous, the exact reason for its form and the consequences that
its solution would have for the initial goal are not so well known in the
mathematician community. In this seminar, I will describe the thrilling
story of these origins: where the problem came from, would it still be
useful for that task, and what is left to do. I will probably talk a lot
and show very few formulas, and I will also present some open problems.

**Adrian Ebert**(RICAM)

*Construction of (polynomial) lattice rules by smoothness-independent component-by-component digit-by-digit constructions*

Abstract

In this talk, we introduce component-by-component digit-by-digit algorithms (CBC-DBD) for the construction of (polynomial) lattice rules in weighted Korobov/Walsh spaces with prescribed decay of the involved series coefficients and associated smoothness $\alpha > 1$. The presented methods are extensions of a construction algorithm established by Korobov in [1] to the modern quasi-Monte Carlo (QMC) setting. We show that the introduced CBC-DBD algorithms construct QMC rules with $N = 2^n$ points which achieve the almost optimal worst-case error convergence rates in the studied function spaces. Due to the used quality functions, the algorithms can construct good (polynomial) lattice rules independent of the smoothness α of the respective function class. Furthermore, we derive suitable conditions on the weights under which the mentioned error bounds are independent of the dimension. The presented algorithms can be implemented in a fast manner such that the construction only requires $O(sN \ln N )$ operations, where $N = 2^n$ is the number of lattice points and s denotes the dimension. We stress that these fast constructions achieve this complexity without the use of fast Fourier transformations (FFTs), as in, e.g., [2]. We present extensive numerical results which confirm our theoretical findings.

Joint research with Peter Kritzer, Dirk Nuyens, Onyekachi Osisiogu, and Tetiana Stepaniuk.

[1] N.M. Korobov. On the computation of optimal coefficients. Dokl. Akad. Nauk SSSR, 26:590–593. 1982.

[2] D. Nuyens, R. Cools. Fast component-by-component construction of rank-1 lattice rules with a non-prime number of points. J. Complexity 22(1), 4–28. 2006.

Joint research with Peter Kritzer, Dirk Nuyens, Onyekachi Osisiogu, and Tetiana Stepaniuk.

[1] N.M. Korobov. On the computation of optimal coefficients. Dokl. Akad. Nauk SSSR, 26:590–593. 1982.

[2] D. Nuyens, R. Cools. Fast component-by-component construction of rank-1 lattice rules with a non-prime number of points. J. Complexity 22(1), 4–28. 2006.

Slides: [pdf]

Video: [mp4]

**Alexey Glazyrin**(UT Rio Grande Valley)

*Mapping to the space of spherical harmonics*

Abstract

For a variety of problems for point configurations in spheres, the
space of spherical harmonics plays an important role. In this talk, we will
discuss maps from point configurations to the space of spherical harmonics.
Such maps can be used for finding bounds on packings, energy bounds, and
constructing new configurations. We will explain classical results from this
perspective and prove several new bounds. Also we will show a new short proof
for the kissing number problem in dimension 3.

Slides: [pdf]

Video: [mp4]

**Alexander Barg**(U of Maryland)

*Stolarsky's invariance principle for the Hamming space*

Abstract

Stolarsky's invariance principle has enjoyed considerable
attention in the literature in the last decade. In this talk we study an
analog of Stolarsky's identity in finite metric spaces with an emphasis on
the Hamming space. We prove several bounds on the spherical discrepancy of
binary codes and identify some discrepancy minimizing configurations. We
also comment on the connection between the problem of minimizing the
discrepancy and the general question of locating minimum-energy
configurations in the space. The talk is based on arXiv:2005.12995 and
arXiv:2007.09721 (joint with Maxim Skriganov).

Slides: [pdf]

Video: [mp4]

**Paul Leopardi**(Australian National U)

*Diameter bounded equal measure partitions of Ahlfors regular metric measure spaces*

Abstract

The algorithm devised by Feige and Schechtman for partitioning
higher dimensional spheres into regions of equal measure and
small diameter is combined with David and Christ's construction
of dyadic cubes to yield a partition algorithm suitable to any
connected Ahlfors regular metric measure space of finite
measure. This is joint work with Giacomo Gigante of the
University of Bergamo.

Slides: [pdf]

Paper: [arXiv] G. Gigante and P. Leopardi, "Diameter bounded equal measure partitions of Ahlfors regular metric measure spaces", Discrete and Computational Geometry, 57 (2), 2017, pp. 419–430.

Video: [mp4]

**Laurent Bétermin**(U of Vienna)

*Theta functions, ionic crystal energies and optimal lattices*

Abstract

The determination of minimizing structures for pairwise interaction
energies is a very challenging crystallization problem. The goal of this talk
is to present recent optimality results among charges and lattice structures
obtained with Markus Faulhuber (University of Vienna) and Hans Knüpfer
(University of Heidelberg). The central object of these works is the heat
kernel associated to a lattice, also called lattice theta function. Several
connections will be showed between interaction energies and theta functions in
order to study the following problems:
- Born’s Conjecture: how to distribute charges on a fixed lattice in order to
minimize the associated Coulombian energy? In the simple cubic case, Max Born
conjectured that the alternation of charges +1 and -1 (i.e. the rock-salt
structure of NaCl) is optimal. The proof of this conjecture obtained with Hans
Knüpfer will be briefly discussed as well as its generalization to other
lattices and energies.
- stability of the rock-salt structure: what could be conditions on
interaction potentials such that the minimal energies among charges and
lattices has a rock-salt structure? Many results, both theoretical and
numerical and obtained with Markus Faulhuber and Hans Knüpfer, will be
presented.
- maximality of the triangular lattices among lattices with alternation of
charges: we will present this new universal optimality among lattices obtained
with Markus Faulhuber.

Slides: [pdf]

Video: [mp4]

## Summer 2020

Date | Speaker | Affiliation | Title |
---|---|---|---|

Jun 3 | Ujué Etayo | TU Graz | Astounding connections of the logarithmic energy on the sphere |

Jun 10 | Josiah Park | Georgia Tech | Optimal measures for three-point energies and semidefinite programming |

Jun 17 | Maria Dostert | EPFL | Semidefinite programming bounds for the average kissing number |

Jun 24 | Philippe Moustrou | The Arctic University of Norway | Exact semidefinite programming bounds for packing problems |

Jul 1 | David de Laat | TU Delft | High-dimensional sphere packing and the modular bootstrap |

Jul 8 | Matthew de Courcy-Ireland | EPFL | Lubotzky-Phillips-Sarnak points on a sphere |

Jul 17 | Mateus Sousa | LMU München | Uncertainty principles, interpolation formulas and packing problems |

Jul 22 | Giuseppe Negro | U of Birmingham | Sharp estimates for the wave equation via the Penrose transform |

Jul 29 | Tetiana Stepaniuk | U of Lübeck | Estimates for the discrete energies on the sphere |

Jul 31 | Mathias Sonnleitner | JKU Linz | Uniform distribution on the sphere and the isotropic discrepancy of lattice point sets |

Aug 5 | Oscar Quesada | IMPA | Developments on the Fourier sign uncertainty principle |

Aug 12 | Louis Brown | Yale | Positive-definite functions, exponential sums and the greedy algorithm: a curious phenomenon |

Aug 14 | Julian Hofstadler | JKU Linz | On a subsequence of random points |

Aug 19 | Felipe Gonçalves | U of Bonn | Sign uncertainty |

Aug 28 | David Krieg | JKU Linz | Order-optimal point configurations for function approximation |

**Ujué Etayo**(TU Graz)

*Astounding connections of the logarithmic energy on the sphere*

Abstract

During this talk we will present different problems that are
somehow related to the following one: find the minimum value of the
logarithmic energy of a set of N points on the sphere of dimension 2. This
late problem has been studied for years, a computational version of it can
be found as Problem Number 7 of Steve Smale list "Mathematical Problems for
the Next Century". This computational version of the problem was proposed
after Smale and Shub found out a beautiful relation between minimizers of
the logarithmic energy and well conditioned polynomials. Working on this
relation, we are able to relate these two concepts to yet a new one: a
sharp Bombieri type inequality for univariate polynomials. The problem can
also be rewritten as a facility location problem, as proved by Beltrán,
since the logarithmic energy is just a normalization of the Green function
for the Laplacian on the sphere.

Slides: [pdf]

Video: [mp4]

**Josiah Park**(Georgia Tech)

*Optimal measures for three-point energies and semidefinite programming*

Abstract

Given a potential function of three vector arguments, \(f(x,y,z)\), which is \(O(n)\)-invariant,
\(f(Qx,Qy,Qz)=f(x,y,z)\) for all \(Q\) orthogonal, we find that surface measure minimizes those
interaction energies of the form \(\int\int\int f(x,y,z) d\mu(x)d\mu(y)d\mu(z)\) over the sphere
whenever the potential function satisfies a positive definiteness criteria. We use
semidefinite programming bounds to determine optimizing probability measures for other
energies. This latter approach builds on previous use of such bounds in the discrete setting
by Bachoc-Vallentin, Cohn-Woo, and Musin, and is successful for kernels which can be shown to
have expansions in a particular basis, for instance certain symmetric polynomials in inner
products \(u=\langle x,y \rangle\), \(v=\langle y,z\rangle\), and \(t=\langle z, x \rangle\). For
other symmetric kernels we pose conjectures on the behavior of optimizers, partially inferred
through numerical studies. This talk is based on joint work with Dmitriy Bilyk, Damir
Ferizovic, Alexey Glazyrin, Ryan Matzke, and Oleksandr Vlasiuk.

**Maria Dostert**(EPFL)

*Semidefinite programming bounds for the average kissing number*

Abstract

The average kissing number of $\mathbb R^n $ is the supremum of the average degrees of contact graphs of packings of finitely many balls (of any radii) in
$ \mathbb R^n.$
In this talk I will provide an upper bound for the average kissing number
based on semidefinite programming that improves previous bounds in dimensions
$3,\ldots, 9$.
A very simple upper bound for the average kissing number is twice the kissing
number; in dimensions $6,\ldots, 9$ our new bound is the first to improve on
this simple upper bound. This is a joined work with Alexander Kolpakov and Fernando
Mário de Oliveira Filho.

Slides: [pdf]

Video: [mp4]

Paper: [arXiv]

**Philippe Moustrou**(The Arctic University of Norway)

*Exact semidefinite programming bounds for packing problems*

Abstract

(Joint work with Maria Dostert and David de Laat.)

In the first part of the talk, we present how semidefinite programming methods can provide upper bounds for various geometric packing problems, such as kissing numbers, spherical codes, or packings of spheres into a larger sphere. When these bounds are sharp, they give additional information on optimal configurations, that may lead to prove the uniqueness of such packings. For example, we show that the lattice $E_8$ is the unique solution for the kissing number problem on the hemisphere in dimension 8.

However, semidefinite programming solvers provide approximate solutions, and some additional work is required to turn them into an exact solution, giving a certificate that the bound is sharp. In the second part of the talk, we explain how, via our rounding procedure, we can obtain an exact rational solution of semidefinite program from an approximate solution in floating point given by the solver.

In the first part of the talk, we present how semidefinite programming methods can provide upper bounds for various geometric packing problems, such as kissing numbers, spherical codes, or packings of spheres into a larger sphere. When these bounds are sharp, they give additional information on optimal configurations, that may lead to prove the uniqueness of such packings. For example, we show that the lattice $E_8$ is the unique solution for the kissing number problem on the hemisphere in dimension 8.

However, semidefinite programming solvers provide approximate solutions, and some additional work is required to turn them into an exact solution, giving a certificate that the bound is sharp. In the second part of the talk, we explain how, via our rounding procedure, we can obtain an exact rational solution of semidefinite program from an approximate solution in floating point given by the solver.

Slides: [pdf]

Video: [mp4]

**David de Laat**(TU Delft)

*High-dimensional sphere packing and the modular bootstrap*

Abstract

Recently, Hartman, Mazáč, and Rastelli discovered a connection
between the Cohn-Elkies bound for sphere packing and problems in the modular
bootstrap. In this talk I will explain this connection and discuss our
numerical study into high dimensional sphere packing and the corresponding
problems in the modular bootstrap. The numerical results indicate an
exponential improvement over the Kabatianskii-Levenshtein bound. I will also
discuss implied kissing numbers and how these relate to improvements over the
Cohn-Elkies bound.

Joint work with Nima Afkhami-Jeddi, Henry Cohn, Thomas Hartman, and Amirhossein Tajdini.

Joint work with Nima Afkhami-Jeddi, Henry Cohn, Thomas Hartman, and Amirhossein Tajdini.

Slides: [pdf]

Video: [mp4]

**Matthew de Courcy-Ireland**(EPFL)

*Lubotzky-Phillips-Sarnak points on a sphere*

Abstract

We will discuss work of Lubotzky-Phillips-Sarnak on special
configurations of points on the two-dimensional sphere: what these points
achieve, the sense in which it is optimal, and aspects of the construction
that are specific to the sphere.

Slides: [pdf]

Video: [mp4]

**Mateus Sousa**(LMU München)

*Uncertainty principles, interpolation formulas and packing problems*

Abstract

In this talk we will discuss how certain uncertainty principles and
interpolation formulas are connected to packing problems and talk about some
recent developments on these fronts.

Slides: [pdf]

Video: [mp4]

**Giuseppe Negro**(U of Birmingham)

*Sharp estimates for the wave equation via the Penrose transform*

Abstract

In 2004, Foschi found the best constant, and the extremizing
functions, for the Strichartz inequality for the wave equation with data in
the Sobolev space $\dot{H}^{1/2}\times \dot{H}^{-1/2}(\mathbb{R}^3)$. He also formulated a
conjecture, concerning the extremizers to this Strichartz inequality in all
spatial dimensions $d\ge 2$. We disprove such conjecture for even $d$, but we
provide evidence to support it for odd $d$. The proofs use the conformal
compactification of the Minkowski space-time given by the Penrose transform.
Part of this talk is based on joint work with Felipe Gonçalves (Univ. Bonn).

Slides: [pdf]

Video: [mp4]

Paper: [arXiv1] [arXiv2]

**Tetiana Stepaniuk**(U of Lübeck)

*Estimates for the discrete energies on the sphere*

Abstract

We find upper and lower estimate for the discrete energies whose
Legendre-Fourier coefficients decrease to zero approximately as power
functions.

Slides: [pdf]

Video: [mp4]

**Mathias Sonnleitner**(JKU Linz)

*Uniform distribution on the sphere and the isotropic discrepancy of lattice point sets*

Abstract

Aistleitner, Brauchart and Dick showed in 2012 how the spherical cap
discrepancy of mapped point sets may be estimated in terms of their isotropic
discrepancy. We provide a characterization of the isotropic discrepancy of
lattice point sets in terms of the spectral test, the inverse length of the
shortest vector in the corresponding dual lattice. This is used to give a
lower bound on the discrepancy in question. The talk is based on joint work
with F. Pillichshammer.

Slides: [pdf]

Video: [mp4]

Paper: [arXiv]

**Oscar Quesada**(IMPA)

*Developments on the Fourier sign uncertainty principle*

Abstract

Can we control the signs of a function and its Fourier transform,
simultaneously, in an arbitrary way?

An uncertainty principle in Fourier analysis is the answer to this type of question. They lie at the heart of Fourier optimization problems, such as the Cohn-Elkies linear program for sphere packings. We will discuss some answers to this question from a new perspective, and why it might be relevant for problems in diophantine geometry and optimal configurations. (Joint work with Emanuel Carneiro).

An uncertainty principle in Fourier analysis is the answer to this type of question. They lie at the heart of Fourier optimization problems, such as the Cohn-Elkies linear program for sphere packings. We will discuss some answers to this question from a new perspective, and why it might be relevant for problems in diophantine geometry and optimal configurations. (Joint work with Emanuel Carneiro).

Slides: [pdf]

Video: [mp4]

Paper: [arXiv]

**Louis Brown**(Yale)

*Positive-definite functions, exponential sums and the greedy algorithm: a curious phenomenon*

Abstract

We describe a curious dynamical system that results in sequences of
real numbers in $[0,1]$ with seemingly remarkable properties. Let the even
function $f:\mathbb{T} \rightarrow \mathbb{R}$ satisfy $\widehat{f}(k) \geq
c|k|^{-2}$ and define a sequence via
$x_n = \arg\min_x \sum_{k=1}^{n-1}{f(x-x_k)}$.

Such greedy sequences seem to be astonishingly regularly distributed in various ways. We explore this, and generalize the algorithm (and results on it) to higher-dimensional manifolds, where the setting is even nicer.

Such greedy sequences seem to be astonishingly regularly distributed in various ways. We explore this, and generalize the algorithm (and results on it) to higher-dimensional manifolds, where the setting is even nicer.

Slides: [pdf]

Video: [mp4]

Paper: [arXiv]

**Julian Hofstadler**(JKU Linz)

*On a subsequence of random points*

Abstract

We want to study the ideas of R. Dwivedi, O. N. Feldheim, O. Guri-Gurevich and A. Ramdas from their paper 'Online thinning in reducing discrepancy', where they give a criterion for choosing points of a random sequence. This technique, called thinning, shall improve the distribution of random points, and we also want to discuss their attempt to create thinned samples with small discrepancy.

Slides: [pdf]

Video: [mp4]

**Felipe Gonçalves**(U of Bonn)

*Sign uncertainty*

Abstract

We will talk about recent developments of the sign uncertainty
principle and its relation with sphere packing bounds and spherical designs.
This is joint work with J. P. Ramos and D. Oliveira e Silva.

Slides: [pdf]

Video: [mp4]

Paper: [arXiv]

**David Krieg**(JKU Linz)

*Order-optimal point configurations for function approximation*

Abstract

We show that independent and uniformly distributed sampling points are as good
as optimal sampling points for the approximation (and integration) of
functions from the Sobolev space $W_p^s(\Omega)$ on domains $\Omega\subset
\mathbb{R}^d$ in the $L_q(\Omega)$-norm whenever $q<p$, where we take $q=1$ if
we only want to compute the integral. In the case $q\ge p$ there is a loss of
a logarithmic factor. More generally, we characterize the quality of arbitrary
sampling points $P\subset \Omega$ via the $L_\gamma(\Omega)$-norm of the
distance function ${\rm dist}(\cdot,P)$, where $\gamma=s(1/q-1/p)_+^{-1}$.
This improves upon previous characterizations based on the covering radius of
$P$. This is joint work with M. Sonnleitner.

Video: [mp4]

Slides: [pdf]