# Schedule for: 19w5028 - Helly and Tverberg type Theorems

Beginning on Sunday, October 6 and ending Friday October 11, 2019

All times in Oaxaca, Mexico time, CDT (UTC-5).

Sunday, October 6 | |
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14:00 - 23:59 | Check-in begins (Front desk at your assigned hotel) |

19:30 - 22:00 | Dinner (Restaurant Hotel Hacienda Los Laureles) |

20:30 - 21:30 | Informal gathering (Hotel Hacienda Los Laureles) |

Monday, October 7 | |
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07:30 - 09:00 | Breakfast (Restaurant at your assigned hotel) |

09:30 - 09:45 | Introduction and Welcome (Conference Room San Felipe) |

09:45 - 10:30 |
Jesus De Loera: Data Classification Algorithms and Tverberg-type theorems ↓ The classical Tverberg's theorem says that a set with sufficiently many points in $R^d$ can always be partitioned into m parts so that the (m - 1)-simplex is the (nerve) intersection pattern of the convex hulls of the parts. Motivated by questions from the performance of data
classification algorithms such as multi-class logistic regression method, we investigated other version Tverberg. Our main results demonstrate that Tverberg's theorem is but a special case of a much more general situation. Given sufficiently many points, any tree or cycle, can also be induced by at least one partition of the point set. The proofs require a deep investigation of oriented matroids and order types.
We also present new results on the distribution of simplicial complexes arising from the classification of data.
(Joint work with Deborah Oliveros, Tommy Hogan, Dominic Yang (supported by NSF).) (Conference Room San Felipe) |

10:30 - 11:00 | Coffee Break (Conference Room San Felipe) |

11:00 - 11:45 |
Leonardo Ignacio Martínez Sandoval: On a hypersimplicial Van Kampen-Flores theorem ↓ The convex dimension of a k-uniform hypergraph is the smallest dimension d for which there is an injective mapping of its vertices into R^d such that the set of k-barycenters of all hyperedges is in convex position.
We completely determine the convex dimension of complete k-uniform hypergraphs. This settles an open question by Halman, Onn and Rothblum, who solved the problem for complete graphs. We also provide lower and upper bounds for the extremal problem of estimating the maximal number of hyperedges of k-uniform hypergraphs on n vertices with convex dimension d.
To prove these results we restate them in terms of affine projections of the hypersimplex that preserve its vertices. More generally, we study projections that preserve higher dimensional skeleta. In particular, we obtain a hypersimplicial generalization of the linear van Kampen-Flores theorem: for each n, k and i we determine onto which dimensions can the (n,k)-hypersimplex be linearly projected while preserving its i-skeleton. (Conference Room San Felipe) |

11:55 - 12:15 |
Jorge Ramirez Alfonsin: On a scissors congruence phenomenon for some polytopes ↓ The scissors congruence conjecture for the unimodular group is an analogue of Hilbert?s third problem, for the equidecomposability of polytopes. In this talk, we present a proof of this conjecture for polytopes naturally associated to graphs whose vertices have degree one or three. The key ingredient in the proof is the nearest neighbor interchange on graphs and a naturally arising piecewise unimodular transformation. We also discuss some Ehrhart quasi-polynomials results for this class of polytopes (joint work with C.G. Fernandes, J.C. de Pina and S. Robins). (Conference Room San Felipe) |

13:20 - 13:30 | Group Photo (Hotel Hacienda Los Laureles) |

13:30 - 15:00 | Lunch (Restaurant Hotel Hacienda Los Laureles) |

15:15 - 16:00 |
Florian Frick: Intersection patterns in good covers. ↓ What are the possible Venn diagrams we can draw with open sets in Euclidean space if all regions must be contractible? I will show that there is only one type of local obstruction that vanishes if and only if this is possible. If the regions of the diagram must be convex, this question is much more difficult, and stronger obstructions exist. In this case the intersection combinatorics are captured by convex neural codes, and I will present some recent progress towards understanding these codes.
This is joint work with Aaron Chen and Anne Shiu. (Conference Room San Felipe) |

16:00 - 16:30 | Coffee Break (Conference Room San Felipe) |

16:30 - 17:30 |
Alexey Garber: Helly numbers for crystals and cut-and-project sets. ↓ In this talk I'll explain why finite Helly numbers exist for
periodic and certain quasiperiodic sets in Euclidean space of any
dimension though the bounds in the latter case seem to be extremely
non-optimal. I'll also show that for a wider class of Meyer sets Helly
numbers could be infinite. (Conference Room San Felipe) |

19:00 - 21:00 | Dinner (Restaurant Hotel Hacienda Los Laureles) |

Tuesday, October 8 | |
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07:30 - 09:00 | Breakfast (Restaurant at your assigned hotel) |

09:00 - 09:40 | Problem session (Conference Room San Felipe) |

09:45 - 10:30 |
Marton Naszodi: Colorful Helly-type Theorems for Ellipsoids ↓ We prove the following Helly-type result.
Let $\mathcal{C}_1,\ldots,\mathcal{C}_{3d}$ be finite families of convex bodies
in $\mathbb{R}^d$. Assume that for any colorful choice of $2d$ sets, $C_{i_k}\in
\mathcal{C}_{i_k}$ for each $1\leq k\leq 2d$ with
$1\leq i_1< \ldots < i_{2d}\leq 3d$, the intersection
$\bigcap\limits_{k=1}^{2d} C_{i_k}$ contains an ellipsoid of volume at least 1.
Then there is a color class $1\leq i \leq 3d$ such that $\bigcap\limits_{C\in \mathcal{C}_i} C$
contains an ellipsoid of volume at least $d^{-O(d^2)}$.
Joint work with Gábor Damásdi and Viktória Földvári. (Conference Room San Felipe) |

10:30 - 11:00 | Coffee Break (Conference Room San Felipe) |

11:00 - 11:45 |
Liping Yuan: On F-convexity and related problems. ↓ If every $k$-membered subfamily of a family of plane convex bodies has a line transversal, then we say that this family has property $T(k)$. We say that a family $\mathcal{F}$ has property $T-m$, if there
exists a subfamily $\mathcal{G} \subset \mathcal{F}$ with $|\mathcal{F}
- \mathcal{G}| \le m$ admitting a line
transversal. In 2007, Heppes posed the problem whether there exists a convex body $K$
in the plane such that if $\mathcal{F}$ is a finite $T(3)$-family of disjoint translates of $K$,
then $m=3$ is the smallest value for which $\mathcal{F}$ has property $T-m$. In this talk,
we study this open problem {in terms of} finite $T(3)$-families of pairwise disjoint translates of
a regular $2n$-gon $(n \ge 5)$. We find out that, for $5 \le n \le 34$, the family has property $T - 3$;
for $n \ge 35$, the family has property $T - 2$.
This is a joint work with Qingdan Du and Tudor Zamfirescu. (Conference Room San Felipe) |

11:55 - 12:15 |
Javier Bracho: Are we afraid of Projective Geometry? ↓ A few remarks on the history of geometry will lead to a novel approach to how we teach it (and what of it we do).
In particular, conic curves and non euclidean geometries can be seen as a consequence of incidence geometry. (Conference Room San Felipe) |

13:30 - 15:00 | Lunch (Restaurant Hotel Hacienda Los Laureles) |

15:15 - 16:00 |
Pablo Soberón: Exact quantitative Helly theorems. ↓ We will discuss several quantitative Helly theorems, where we characterize families of convex sets whose intersection has a large volume or diameter. The Helly numbers we obtain depend on the complexity of a ?witness set? that guarantees that the intersection is large. For example, our Helly numbers characterize families of convex sets that contain ellipsoids with large volume, or zonotopes with large diameter. The methods we describe work for a wide family of functions and witness sets. We will describe which results can be proven with purely geometric methods and which ones require a bit of topology. (Conference Room San Felipe) |

16:00 - 16:30 | Coffee Break (Conference Room San Felipe) |

16:30 - 17:15 |
Shira Zerbib: The geometry and combinatorics of discrete line segment hypergraphs. ↓ An r-segment hypergraph H is a hypergraph whose edges consist of r consecutive integer points on line segments in ${\mathbb{R}}^2$. We will discuss some recent bounds on the chromatic numbers and covering numbers of r-segment hypergraphs, uncovering several interesting geometric properties in the process.
Joint work with Deborah Oliveros and Chris O'Neill. (Conference Room San Felipe) |

19:00 - 21:00 | Dinner (Restaurant Hotel Hacienda Los Laureles) |

Wednesday, October 9 | |
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07:30 - 09:00 | Breakfast (Restaurant at your assigned hotel) |

09:00 - 09:45 |
Andreas Holmsen: Colorful and Fractional Helly theorems. ↓ I will discuss the combinatorial relationship between the colorful Helly theorem and the fractional Helly theorem, and show some applications in abstract convexity and chi-boundedness of certain hypergraphs. (Conference Room San Felipe) |

09:45 - 10:30 |
Zuzana Patáková: Bounding Radon's number via Betti numbers.} ↓ Radon's theorem is one of the cornerstones of convex geometry.
It implies many of the key results in the area such as Helly's theorem
and, as recently shown by Andreas Holmsen and Dong-Gyu Lee, also its
more robust version, fractional Helly's theorem together with a colorful
strengthening of Helly's theorem. Consequently, this yields an existence
of weak epsilon nets and a (p,q)-theorem.
We show that we can obtain these results even without assuming
convexity, replacing it with very weak topological conditions.
Moreover, using the recent result of myself and Gil Kalai, we manage to
bring the fractional Helly number for open sets in the plane or on a
surface down to three. This also settles a conjecture of Andreas
Holmsen, Minki Kim, and Seunghun Lee about an existence of a
(p,q)-theorem for open subsets of a surface. (Conference Room San Felipe) |

10:30 - 11:00 | Coffee Break (Conference Room San Felipe) |

11:00 - 11:45 |
Pavel Patak: Bounding Radon's number via Betti numbers in the plane. ↓ In her recent result, Zuzana Patáková has shown that
for a finite family $\mathcal F$ of sets in $\mathbb R^d$,
one can use Betti numbers of intersections of subfamilies of $\mathcal
F$, to bound the Radon's number of $\mathcal F$.
The result has interesting consequences, some of them are easy or
standard, other follow from a result of Holmsen and Lee. Let me name
just few:
variants of Helly's, Tverberg's, colorful and fractional Helly theorems,
existence of weak $\varepsilon$-nets, $(p,q)$-theorems, \dots
Nevertheless, the original bounds on Radon's number are too large to be
widely applicable.
We improve the situation in the plane and show how to obtain polynomial bounds.
More generally the result extends to other two-dimensional (pseudo)manifolds. (Conference Room San Felipe) |

11:45 - 12:30 |
Janos Pach: Geometric intersection patterns and order ↓ TBA (Conference Room San Felipe) |

12:30 - 13:30 | Lunch (Restaurant Hotel Hacienda Los Laureles) |

13:30 - 19:00 | Free Afternoon (Oaxaca) |

19:00 - 21:00 | Dinner (Restaurant Hotel Hacienda Los Laureles) |

Thursday, October 10 | |
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07:30 - 09:00 | Breakfast (Restaurant at your assigned hotel) |

09:45 - 10:30 |
Oleg Musin: Borsuk--Ulam type theorem for f--neighbors ↓ We introduce and study a new class of extensions for the Borsuk--Ulam theorem. Our approach is based on the theory of Voronoi diagrams and Delaunay triangulations. One of our main results is as follows.
\begin{thm}\label{corDln1}
Let $S^m$ be a unit sphere in $R^{m+1}$ and let $f: S^m \to R^n$ be a continuous map.
Then there are points $p$ and $q$ in $S^m$ such that
\begin{itemize}
\item $\|p-q\|\ge\sqrt{2\cdot\frac{m+2}{m+1}}$\/{\rm;}
\item $f(p)$ and $f(q)$ lie on the boundary of a closed metric ball $B$ in $R^n$ whose interior does not meet $f(S^m)$.
\end{itemize}
\end{thm}
Note that $\sqrt{2\cdot\frac{m+2}{m+1}}$ is the diameter of a regular simplex inscribed in $S^m$.
Joint paper with Andrey Malyutin. (Conference Room San Felipe) |

10:30 - 11:00 | Coffee Break (Conference Room San Felipe) |

11:00 - 11:45 |
Martin Tancer: Embeddings of k-complexes into 2k-manifolds ↓ Let $K$ be a simplicial $k$-complex and $M$ be a closed PL
$2k$-manifold. Our first aim during the talk is to describe an obstruction
for embeddability of $K$ into $M$ via the intersection form on $M$. For
description of the obstruction, we need a technical condition which is
satisfied, in particular, either if $M$ is $(k-1)$-connected or if $K$ is the
$k$-skeleton of $n$-simplex, for some $n$. Under the technical condition, if $K$
(almost) embeds in $M$, then our obstruction vanishes. In addition, if $M$ is
$(k-1)$-connected and $k \geq 3$, then the obstruction is complete, that is,
we get the reverse implication.
Modulo a recent hard Lefschetz theorem of Adiprasito, a consequence of our
results on the existence and completeness of the obstruction are very good
bounds on the Helly number in a certain Helly type theorem where the
ambient space is a (suitable) manifold. The details will be explained
during the talk.
The talk is based on a joint work with Pavel Paták. (Conference Room San Felipe) |

11:55 - 12:15 |
Efren Morales Amaya: Characterizations of the sphere by means of visual cones: an alternative proof of Matsuura's theorem ↓ In this work we prove that if there exists a point $p\in \mathbb{R}^n$ and a smooth convex body $M$ in $\mathbb{R}^n$, $n\geq 3$, contained in the interior of the unit ball of $\mathbb{R}^n$, such that $M$ looks centrally symmetric, and $p$ appears as the centre, from each point of $\mathbb{S}^{n-1}$, then $M$ is an sphere. Using this result we derived, straightaway, a well known characterization of the sphere due to S. Matsuura (Conference Room San Felipe) |

13:30 - 15:00 | Lunch (Restaurant Hotel Hacienda Los Laureles) |

15:15 - 16:00 |
Luis Montejano: On the isometric hypothesis of Banach ↓ The following is known as the geometric hypothesis of Banach: let V be an m-dimensional Banach space with unit ball B and suppose all n-dimensional subspaces of V are isometric (all the n-sections of B are affinely equivalent). In 1932, Banach conjectured that under this hypothesis V is a Hilbert space (the boundary of B is an ellipsoid). Gromow proved in 1967 that the conjecture is true for n=even and Dvoretzky derived the same conclusion under the hypothesis n=infinity. We prove this conjecture for all positive integers of the form n=4k+1, with the possible exception of 133.
The ingredients of the proof are classical homotopic theory, irreducible representations of the orthogonal group and convex geometry (Conference Room San Felipe) |

16:00 - 16:30 | Coffee Break (Conference Room San Felipe) |

16:30 - 17:15 |
Roman Karasev: Envy-free division using mapping degree. ↓ We discuss some classical problems of mathematical economics, in
particular, so-called envy-free division problems. The classical
approach to some of such problem reduces to considering continuous maps
of a simplex to itself and finding sufficient conditions when this map
hits the center of the simplex. The mere continuity is not sufficient
for such a conclusion, the usual assumption (for example, in the
Knaster--Kuratowski--Mazurkiewicz theorem and the Gale theorem) is a
boundary condition.
We try to replace the boundary condition by a certain equivariance
condition under all permutations, or a weaker condition of
``pseudo-equivariance'', which has a certain real-life meaning for the
problem of partitioning a segment and distributing the parts among the
players. It turns out that we can guarantee the existence of a solution
for the segment partition problem when the number of players is a prime
power; and we may produce instances of the problem without a solution
otherwise. The case of three players was solved previously be
Segal-Halevi, the prime case and the case of four players were solved by
Meunier and Zerbib.
Going back to the true equivariance setting, we provide, in the case
when the number of players is odd and not a prime power, the
counterexamples showing that the topological configuration space / test
map scheme for a wide class of equipartition problems fails. This is
applicable, for example, to building stronger counterexamples for the
topological Tverberg conjecture (in another joint work with Sergey
Avvakumov and Arkadiy Skopenkov).
Joint work with Sergey Avvakumov (Conference Room San Felipe) |

19:00 - 21:00 | Dinner (Restaurant Hotel Hacienda Los Laureles) |

Friday, October 11 | |
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07:30 - 09:00 | Breakfast (Restaurant at your assigned hotel) |

09:45 - 10:30 |
Emo Welzl: Bistellar and Edge Flip Graphs of Triangulations in the Plane - Geometry and Connectivity. ↓ The set of all triangulations of a finite point set in the plane attains structure via flips: The graph, where two triangulations are adjacent if one can be obtained from the other by an elementary flip. This is an edge flip for full triangulations, or a bistellar flip for partial triangulations (where non-extreme points can be skipped).
It is well-known (Lawson, 1972) that both, the edge flip graph and the bistellar flip graph are connected. For n the number of points and general position assumed, we show that the edge flip graph is (n/2 - 2)-connected and the bistellar flip graph is (n-3)-connected. Both bounds are tight.
This matches the situation for regular triangulations (a subset of the partial triangulations), where (n-3)-connectivity was known through the secondary polytope (Gelfand, Kapranov, Zelevinsky, 1990) and Balinski's Theorem. We show that the edge flip graph can be covered by 1-skeletons of polytopes of dimension at least n/2-2 (products of associahedra). Similarly, the bistellar flip graph can be covered by 1-skeletons of polytopes of dimension at least n-3 (products of secondary polytopes).
(covers joint research with Uli Wagner, IST Austria) (Conference Room San Felipe) |

10:30 - 11:00 | Coffee Break (Conference Room San Felipe) |

11:00 - 11:45 |
Imre Bárány: Theorems of Helly and Tverberg without dimension ↓ The Helly theorem in the title says the following. Assume $k < d+1$ and $F$ is a finite family of convex bodies, all contained in the Euclidean unit ball of $R^d$ with the property that every $k$-tuple of sets in $F$ has a point in common. Then there is a point $q$ in $R^d$ which is closer than $1/ \sqrt k$ to every set in $F$. This result has several colourful and fractional variants. Similar versions of Tverberg's theorem and some of their extensions are also established. This is joint work with Karim Adiprasito, Nabil Mustafa, and Tamás Terpai. (Conference Room San Felipe) |

12:00 - 14:00 | Lunch (Restaurant Hotel Hacienda Los Laureles) |