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BEGIN:VEVENT
SUMMARY:Manfred Scheucher (TU Berlin)
DTSTART;VALUE=DATE-TIME:20210121T150000Z
DTEND;VALUE=DATE-TIME:20210121T170000Z
DTSTAMP;VALUE=DATE-TIME:20210419T093853Z
UID:CJCS/1
DESCRIPTION:Title: Usi
ng SAT Solvers in Combinatorics and Geometry\nby Manfred Scheucher (TU
Berlin) as part of Copenhagen-Jerusalem Combinatorics Seminar\n\nAbstract
: TBA\n
LOCATION:https://researchseminars.org/talk/CJCS/1/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Man-Wai Cheung (Harvard University)
DTSTART;VALUE=DATE-TIME:20210128T150000Z
DTEND;VALUE=DATE-TIME:20210128T170000Z
DTSTAMP;VALUE=DATE-TIME:20210419T093853Z
UID:CJCS/2
DESCRIPTION:Title: Com
pactifications of cluster varieties and convexity\nby Man-Wai Cheung (
Harvard University) as part of Copenhagen-Jerusalem Combinatorics Seminar\
n\nAbstract: TBA\n
LOCATION:https://researchseminars.org/talk/CJCS/2/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Bruno Benedetti (U Miami)
DTSTART;VALUE=DATE-TIME:20210204T150000Z
DTEND;VALUE=DATE-TIME:20210204T170000Z
DTSTAMP;VALUE=DATE-TIME:20210419T093853Z
UID:CJCS/3
DESCRIPTION:Title: A d
-dimensional version of interval graphs\, of Hamiltonian paths\, and of b
inomial edge ideals\nby Bruno Benedetti (U Miami) as part of Copenhage
n-Jerusalem Combinatorics Seminar\n\n\nAbstract\nWe study the d-dimensiona
l generalization of three mutually-related\nnotions in graph theory: (unit
)-interval graphs\, Hamiltonian cycles\, and\nbinomial edge ideals.\n\nThi
s is joint work with Matteo Varbaro and Lisa Seccia (arXiv:2101.09243).\n
LOCATION:https://researchseminars.org/talk/CJCS/3/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Sebastian Manecke (University of Frankfurt)
DTSTART;VALUE=DATE-TIME:20210211T150000Z
DTEND;VALUE=DATE-TIME:20210211T160000Z
DTSTAMP;VALUE=DATE-TIME:20210419T093853Z
UID:CJCS/4
DESCRIPTION:Title: Ins
cribable fans\, zonotopes\, and reflection arrangements\nby Sebastian
Manecke (University of Frankfurt) as part of Copenhagen-Jerusalem Combinat
orics Seminar\n\n\nAbstract\nSteiner posed the question if any 3-dimension
al polytope had a\nrealization with vertices on a sphere. Steinitz constru
cted the first\ncounter example and Rivin gave a complete resolution. In\n
dimensions 4 and up\, universality theorems by Mnev/Richter-Gebert\nrender
the question for inscribable combinatorial types hopeless.\n\nHowever\, f
or a given complete fan N\, we can decide in polynomial time\nif there is
an inscribed polytope with normal fan N. Linear\nhyperplane arrangements c
an be realized as normal fans via zonotopes.\nIt turns out that inscribed
zonotopes are rare and in this talk I\nwill focus on the question of class
ifying the corresponding\narrangements. This relates to localizatons and r
estrictions of\nreflection arrangements and Grünbaum's quest for the clas
sification of \nsimplicial arrangements. The talk is based on joint work w
ith Raman\nSanyal.\n
LOCATION:https://researchseminars.org/talk/CJCS/4/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Martin Tancer (Charles University Prague)
DTSTART;VALUE=DATE-TIME:20210218T150000Z
DTEND;VALUE=DATE-TIME:20210218T170000Z
DTSTAMP;VALUE=DATE-TIME:20210419T093853Z
UID:CJCS/5
DESCRIPTION:Title: The
unbearable hardness of unknotting\nby Martin Tancer (Charles Universi
ty Prague) as part of Copenhagen-Jerusalem Combinatorics Seminar\n\n\nAbst
ract\nDuring the talk\, I will sketch a proof that deciding if a diagram o
f the unknot can be untangled using at most k Riedemeister moves (where k
is part of the input) is NP-hard. (This is not the same as the unknot reco
gnition but it reveals some difficulties.) Similar ideas can be also used
for proving that several other similar invariants are NP-hard to recognize
on links.\n\nJoint work with A. de Mesmay\, Y. Rieck and E. Sedgwick.\n
LOCATION:https://researchseminars.org/talk/CJCS/5/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Vasso Petrotou (University of Ioannina)
DTSTART;VALUE=DATE-TIME:20210225T150000Z
DTEND;VALUE=DATE-TIME:20210225T170000Z
DTSTAMP;VALUE=DATE-TIME:20210419T093853Z
UID:CJCS/6
DESCRIPTION:Title: Tom
& Jerry triples\nby Vasso Petrotou (University of Ioannina) as part o
f Copenhagen-Jerusalem Combinatorics Seminar\n\n\nAbstract\nUnprojection i
s a theory due to Reid which constructs and analyses more complicated ring
s from simpler ones. The talk will be about a new format of unprojection w
hich we call Tom & Jerry triples. As an application we will construct two
families of codimension 6 Fano 3-folds in weighted projective space.\n\nWe
will also give a brief introduction to Macaulay2.\n
LOCATION:https://researchseminars.org/talk/CJCS/6/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Irene Parada (TU Eindhoven)
DTSTART;VALUE=DATE-TIME:20210311T150000Z
DTEND;VALUE=DATE-TIME:20210311T170000Z
DTSTAMP;VALUE=DATE-TIME:20210419T093853Z
UID:CJCS/7
DESCRIPTION:Title: Ins
erting edges into simple drawings\nby Irene Parada (TU Eindhoven) as p
art of Copenhagen-Jerusalem Combinatorics Seminar\n\n\nAbstract\nSimple dr
awings of graphs are those in which each pair of edges share at most one p
oint\, either a common endpoint or a proper crossing. Given a simple drawi
ng D of a graph G\, in this talk we consider the problem of inserting a gi
ven set of missing edges (edges of the complement of G) into D such that t
he result is again a simple drawing. We show that it is NP-complete to dec
ide whether one edge can be inserted into a simple drawing. On the positiv
e side\, we present a Fixed-Parameter Tractable (FPT) algorithm for this p
roblem parameterized by the number of crossings that the edge to be insert
ed can have. This algorithm is tight under the Exponential Time Hypothesis
. We also obtain an FPT algorithm for inserting a bounded number of edges
with a bounded number of crossings. In these FPT algorithms\, after workin
g in the drawing\, the problem boils down to finding an algorithm for a la
beled abstract graph. To obtain these FPT algorithms we use different tool
s including the sunflower lemma\, representative families for matroids\, a
nd Courcelle's theorem. These techniques\, useful in many parameterized al
gorithms\, will be briefly introduced during the talk.\n
LOCATION:https://researchseminars.org/talk/CJCS/7/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Pilar Cano (Université Libre de Bruxelles)
DTSTART;VALUE=DATE-TIME:20210304T150000Z
DTEND;VALUE=DATE-TIME:20210304T160000Z
DTSTAMP;VALUE=DATE-TIME:20210419T093853Z
UID:CJCS/8
DESCRIPTION:Title: Fli
ps in higher order Delaunay triangulations\nby Pilar Cano (Université
Libre de Bruxelles) as part of Copenhagen-Jerusalem Combinatorics Seminar
\n\n\nAbstract\nWe study the flip graph of higher order Delaunay triangula
tions. A triangulation of a set S of n points in the plane is order-k Dela
unay if for every triangle its circumcircle encloses at most k points of S
. The flip graph of S has one vertex for each possible triangulation of S\
, and an edge connecting two vertices when the two corresponding triangula
tions can be transformed into each other by a flip (i.e.\, exchanging the
diagonal of a convex quadrilateral by the other one). The flip graph is an
essential structure in the study of triangulations\, but until now it had
been barely studied for order-k Delaunay triangulations. In this work we
show that\, even though the order-k flip graph might be disconnected for k
≥ 3\, any order-k triangulation can be transformed into some other orde
r-k triangulation by at most k − 1 flips\, such that the intermediate tr
iangulations are of order 2k − 2\, in the following settings: (1) for an
y k ≥ 0 when S is in convex position\, and (2) for any k ≤ 5 and any p
oint set S. Our results imply that the flip distance between two order-k t
riangulations is O(kn)\, as well as an efficient enumeration algorithm.\n
LOCATION:https://researchseminars.org/talk/CJCS/8/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Lukas Kühne (Max Planck Institute of Mathematics in the Sciences)
DTSTART;VALUE=DATE-TIME:20210318T151500Z
DTEND;VALUE=DATE-TIME:20210318T170000Z
DTSTAMP;VALUE=DATE-TIME:20210419T093853Z
UID:CJCS/9
DESCRIPTION:Title: Inv
estigating Terao's freeness conjecture with computer algebra\nby Lukas
Kühne (Max Planck Institute of Mathematics in the Sciences) as part of C
openhagen-Jerusalem Combinatorics Seminar\n\n\nAbstract\nMotivated by sing
ularity theory\, Hiroaki Terao introduced a module of logarithmic derivati
ons associated with a hyperplane arrangement. This talk is concerned with
Terao’s freeness conjecture which asserts that the freeness of this deri
vation module is determined by the underlying combinatorics of the arrange
ment.\n\nTo investigate this conjecture\, we have enumerated all matroids
of rank 3 with up to 14 hyperplanes whose characteristic polynomial splits
over the integers and saved it in a public database. Using the GAP packag
e ZariskiFrames we have computed the moduli space and the free locus of th
e derivation module of each of these matroids as a quasi-affine set. As th
e main result\, this yields a computational proof of Terao’s freeness co
njecture for rank 3 arrangements with up to 14 hyperplanes in arbitrary ch
aracteristic.\n\nIn this talk\, I will explain the background of this conj
ecture without assuming prior knowledge and demonstrate the database and h
ighlights of the computations.\n\nThis talk is based on joint work with Mo
hamed Barakat\, Reimer Behrends\, Christopher Jefferson\, and Martin Lerne
r.\n
LOCATION:https://researchseminars.org/talk/CJCS/9/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Pablo Soberon (CUNY)
DTSTART;VALUE=DATE-TIME:20210325T150000Z
DTEND;VALUE=DATE-TIME:20210325T170000Z
DTSTAMP;VALUE=DATE-TIME:20210419T093853Z
UID:CJCS/10
DESCRIPTION:Title: Tv
erberg's theorem beyond prime powers\nby Pablo Soberon (CUNY) as part
of Copenhagen-Jerusalem Combinatorics Seminar\n\n\nAbstract\nTverberg-type
theory aims to establish sufficient conditions for a simplicial complex $
\\Sigma$ such that every continuous map $f:\\Sigma \\to \\mathbb{R}^d$ map
s $q$ points from pairwise disjoint faces to the same point in $\\mathbb{R
}^d$. Such results are plentiful for $q$ a prime power. However\, for $q
$ with at least two distinct prime divisors\, results that guarantee the e
xistence of $q$-fold points of coincidence are non-existent— aside from
immediate corollaries of the prime power case. Here we present a general
method that yields such results beyond the case of prime powers. Joint wo
rk with Florian Frick.\n
LOCATION:https://researchseminars.org/talk/CJCS/10/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Torsten Mütze (University of Warwick)
DTSTART;VALUE=DATE-TIME:20210506T140000Z
DTEND;VALUE=DATE-TIME:20210506T160000Z
DTSTAMP;VALUE=DATE-TIME:20210419T093853Z
UID:CJCS/11
DESCRIPTION:by Torsten Mütze (University of Warwick) as part of Copenhage
n-Jerusalem Combinatorics Seminar\n\nInteractive livestream: https://ucph-
ku.zoom.us/j/69204766431\nAbstract: TBA\n
LOCATION:https://researchseminars.org/talk/CJCS/11/
URL:https://ucph-ku.zoom.us/j/69204766431
END:VEVENT
BEGIN:VEVENT
SUMMARY:Sarah Peluse (Princeton)
DTSTART;VALUE=DATE-TIME:20210408T140000Z
DTEND;VALUE=DATE-TIME:20210408T160000Z
DTSTAMP;VALUE=DATE-TIME:20210419T093853Z
UID:CJCS/12
DESCRIPTION:Title: Mo
dular zeros in the character table of the symmetric group\nby Sarah Pe
luse (Princeton) as part of Copenhagen-Jerusalem Combinatorics Seminar\n\n
\nAbstract\nIn 2017\, Miller conjectured\, based on computational evidence
\, that for any fixed prime $p$ the density of entries in the character ta
ble of $S_n$ that are divisible by $p$ goes to $1$ as $n$ goes to infinity
. I’ll describe a proof of this conjecture\, which is joint work with K.
Soundararajan. I will also discuss the (still open) problem of determinin
g the asymptotic density of zeros in the character table of $S_n$\, where
it is not even clear from computational data what one should expect.\n
LOCATION:https://researchseminars.org/talk/CJCS/12/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Stefan Felsner (TU Berlin)
DTSTART;VALUE=DATE-TIME:20210422T140000Z
DTEND;VALUE=DATE-TIME:20210422T160000Z
DTSTAMP;VALUE=DATE-TIME:20210419T093853Z
UID:CJCS/13
DESCRIPTION:by Stefan Felsner (TU Berlin) as part of Copenhagen-Jerusalem
Combinatorics Seminar\n\nInteractive livestream: https://ucph-ku.zoom.us/j
/69204766431\nAbstract: TBA\n
LOCATION:https://researchseminars.org/talk/CJCS/13/
URL:https://ucph-ku.zoom.us/j/69204766431
END:VEVENT
BEGIN:VEVENT
SUMMARY:Sean Eberhard (University of Cambridge)
DTSTART;VALUE=DATE-TIME:20210513T140000Z
DTEND;VALUE=DATE-TIME:20210513T160000Z
DTSTAMP;VALUE=DATE-TIME:20210419T093853Z
UID:CJCS/14
DESCRIPTION:by Sean Eberhard (University of Cambridge) as part of Copenhag
en-Jerusalem Combinatorics Seminar\n\nInteractive livestream: https://ucph
-ku.zoom.us/j/69204766431\nAbstract: TBA\n
LOCATION:https://researchseminars.org/talk/CJCS/14/
URL:https://ucph-ku.zoom.us/j/69204766431
END:VEVENT
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