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PRODID:-//Discrete Mathematics Group - ECPv6.15.20//NONSGML v1.0//EN
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X-ORIGINAL-URL:https://dimag.ibs.re.kr
X-WR-CALDESC:Events for Discrete Mathematics Group
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BEGIN:VTIMEZONE
TZID:Asia/Seoul
BEGIN:STANDARD
TZOFFSETFROM:+0900
TZOFFSETTO:+0900
TZNAME:KST
DTSTART:20200101T000000
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BEGIN:VEVENT
DTSTART;TZID=Asia/Seoul:20210601T163000
DTEND;TZID=Asia/Seoul:20210601T173000
DTSTAMP:20260419T224838
CREATED:20210517T132410Z
LAST-MODIFIED:20240707T081332Z
UID:4091-1622565000-1622568600@dimag.ibs.re.kr
SUMMARY:Doowon Koh (고두원)\, Mattila-Sjölin type functions: A finite field model
DESCRIPTION:Let $\mathbb{F}_q$ be a finite field of order $q$ which is a prime power. In the finite field setting\, we say that a function $\phi\colon \mathbb{F}_q^d\times \mathbb{F}_q^d\to \mathbb{F}_q$ is a Mattila-Sjölin type function in $\mathbb{F}_q^d$ if for any $E\subset \mathbb{F}_q^d$ with $|E|\gg q^{\frac{d}{2}}$\, we have $\phi(E\, E)=\mathbb{F}_q$. The main purpose of this talk is to present the existence of such a function. More precisely\, we will construct a concrete function $\phi: \mathbb{F}_q^4\times \mathbb{F}_q^4\to \mathbb{F}_q$ with $q\equiv 3 \mod{4}$ such that if $E\subset \mathbb F_q^4$ with $|E|>q^2\,$ then $\phi(E\,E)=\mathbb F_q$. This is a joint work with Daewoong Cheong\, Thang Pham\, and Chun-Yen Shen.
URL:https://dimag.ibs.re.kr/event/2021-06-01/
LOCATION:Room B232\, IBS (기초과학연구원)
CATEGORIES:Discrete Math Seminar
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Asia/Seoul:20210608T150000
DTEND;TZID=Asia/Seoul:20210608T160000
DTSTAMP:20260419T224838
CREATED:20210514T092018Z
LAST-MODIFIED:20240707T081316Z
UID:4080-1623164400-1623168000@dimag.ibs.re.kr
SUMMARY:O-joung Kwon (권오정)\, Classes of intersection digraphs with good algorithmic properties
DESCRIPTION:An intersection digraph is a digraph where every vertex $v$ is represented by an ordered pair $(S_v\, T_v)$ of sets such that there is an edge from $v$ to $w$ if and only if $S_v$ and $T_w$ intersect. An intersection digraph is reflexive if $S_v\cap T_v\neq \emptyset$ for every vertex $v$. Compared to well-known undirected intersection graphs like interval graphs and permutation graphs\, not many algorithmic applications on intersection digraphs have been developed. \nMotivated by the successful story on algorithmic applications of intersection graphs using a graph width parameter called mim-width\, we introduce its directed analogue called `bi-mim-width’ and prove that various classes of reflexive intersection digraphs have bounded bi-mim-width. In particular\, we show that as a natural extension of $H$-graphs\, reflexive $H$-digraphs have linear bi-mim-width at most $12|E(H)|$\, which extends a bound on the linear mim-width of $H$-graphs [On the Tractability of Optimization Problems on $H$-Graphs. Algorithmica 2020] \nFor applications\, we introduce a novel framework of directed versions of locally checkable problems\, that streamlines the definitions and the study of many problems in the literature and facilitates their common algorithmic treatment. We obtain unified polynomial-time algorithms for these problems on digraphs of bounded bi-mim-width\, when a branch decomposition is given. Locally checkable problems include Kernel\, Dominating Set\, and Directed $H$-Homomorphism. \nThis seminar is a part of the\n“Round the World Relay in Combinatorics”\non June 8\, 2021.\nhttp://people.maths.ox.ac.uk/scott/relay.htm \n\n2:00 UTC\, 11:00 KST Melbourne (Australia) Monash University\nDavid Wood (Monash University)\, Universality in Minor-Closed Graph Classes\n3:00 UTC\, 12:00 KST Shanghai (China) Shanghai Center for Mathematical Sciences\nBaogang Xu (Nanjing Normal University\, China)\, On coloring of graphs of girth 2l+1 without longer odd holes\n4:00 UTC\, 13:00 KST Auckland (New Zealand) The University of Auckland\nJeroen Schillewaert (University of Auckland)\, Constructing highly regular expanders from hyperbolic Coxeter groups\n5:00 UTC\, 14:00 KST Sydney (Australia) Combinatorial Mathematics Society of Australasia\nGordon Royle (University of Western Australia (UWA))\, Real chromatic roots of planar graphs\n6:00 UTC\, 15:00 KST Daejeon (Korea) IBS Discrete Mathematics Group\nO-joung Kwon (Incheon National University and IBS Discrete Mathematics Group)\, Classes of intersection digraphs with good algorithmic properties\n7:00 UTC\, 16:00 KST Krakow (Poland) Jagiellonian University\nBartosz Walczak (Jagiellonian)\, Coloring polygon visibility graphs and their generalizations\n8:00 UTC\, 17:00 KST Glasgow (UK) University of Glasgow\nHeng Guo (University of Edinburgh)\, A Markov chain approach towards the sampling Lovász local lemma\n9:00 UTC\, 18:00 KST London (UK) London School of Economics\nAnnika Heckel (LMU)\, How does the chromatic number of a random graph vary?\n10:00 UTC\, 19:00 KST Moscow (Russia) Moscow Institute of Physics and Technology\nNoga Alon (Princeton and Tel Aviv)\n11:00 UTC\, 20:00 KST Budapest (Hungary) Hungarian Academy of Sciences + Eötvös Loránd University\nLászló Lovász (Eotvos University\, Budapest)\n12:00 UTC\, 21:00 KST Bordeaux (France) LaBRI\nCarla Groenland (Utrecht University)\, Universal Graphs and Labelling Schemes\n13:00 UTC\, 22:00 KST New York (USA) City University of New York + Montclair State University + Hofstra University\nDeepak Bal (Montclair State University)\n14:00 UTC\, 23:00 KST Prague (Czech) Czech Academy of Sciences + Czech Technical University + London School of Economics\nDhruv Mubayi (University of Illinois at Chicago)\, The feasible region of hypergraphs\n15:00 UTC\, 00:00 KST Brno (Czech) Masaryk University\nJames Davies (University of Waterloo)\, Colouring circle graphs and their generalisations\n16:00 UTC\, 01:00 KST Oxford (UK) University of Oxford\nJacob Fox (Stanford University)\, Removal lemmas\n17:00 UTC\, 02:00 KST Columbus (USA) Ohio State University\nAllan Sly (Princeton University)\n18:00 UTC\, 03:00 KST Rio (Brazil) Instituto de Matemática Pura e Aplicada\nMarcelo Campos (IMPA)\, The singularity probability of a random symmetric matrix is exponentially small\n19:00 UTC\, 04:00 KST Atlanta (USA) Georgia Institute of Technology\nJim Geelen (University of Waterloo)\, Homomorphisms and colouring for graphs and binary matroids\n20:00 UTC\, 05:00 KST Santiago (Chile) Universidad de Chile\nDavid Conlon (Caltech)\n21:00 UTC\, 06:00 KST Burnaby (Canada) Simon Fraser University\nFan Chung (UCSD)\, Trees and forests in Green’s functions of a graph\n22:00 UTC\, 07:00 KST Victoria (Canada) University of Victoria\nAndrew Suk (UCSD)\, Turán-type problems for point-line incidences\n23:00 UTC\, 08:00 KST Fairbanks (USA) University of Alaska\nRon Gould (Emory)\, Chorded cycles\n\n 
URL:https://dimag.ibs.re.kr/event/2021-06-08/
LOCATION:Zoom ID: 875 9395 3555 (relay) [CLOSED]
CATEGORIES:Discrete Math Seminar
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Asia/Seoul:20210622T163000
DTEND;TZID=Asia/Seoul:20210622T173000
DTSTAMP:20260419T224838
CREATED:20210430T062352Z
LAST-MODIFIED:20240705T184213Z
UID:4028-1624379400-1624383000@dimag.ibs.re.kr
SUMMARY:Hongseok Yang (양홍석)\, DAG-symmetries and Symmetry-Preserving Neural Networks
DESCRIPTION:The preservation of symmetry is one of the key tools for designing data-efficient neural networks. A representative example is convolutional neural networks (CNNs); they preserve translation symmetries\, and this symmetry preservation is often attributed to their success in real-world applications. In the machine-learning community\, there is a growing body of work that explores a new type of symmetries\, both discrete and continuous\, and studies neural networks that preserve those symmetries. \nIn this talk\, I will explain what I call DAG-symmetries and our preliminary results on the shape of neural networks that preserve these symmetries. DAG-symmetries are finite variants of DAG-exchangeability developed by Jung\, Lee\, Staton\, and Yang (2020) in the context of probabilistic symmetries. Using these symmetries\, we can express that when a neural network works on\, for instance\, sets of bipartite graphs whose edges are labelled with reals\, the network depends on neither the order of elements in the set nor the identities of vertices of the graphs. I will explain how a group of specific DAG-symmetries is constructed by applying a form of wreath product over a given finite DAG. Then\, I will explain what linear layers of neural networks preserving these symmetries should look like. \nThis is joint work with Dongwoo Oh.
URL:https://dimag.ibs.re.kr/event/2021-06-22/
LOCATION:Room B232\, IBS (기초과학연구원)
CATEGORIES:Discrete Math Seminar
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BEGIN:VEVENT
DTSTART;TZID=Asia/Seoul:20210629T163000
DTEND;TZID=Asia/Seoul:20210629T173000
DTSTAMP:20260419T224838
CREATED:20210614T232723Z
LAST-MODIFIED:20240707T081306Z
UID:4251-1624984200-1624987800@dimag.ibs.re.kr
SUMMARY:Jeong Ok Choi (최정옥)\, Invertibility of circulant matrices of arbitrary size
DESCRIPTION:In this talk\, we present sufficient conditions to guarantee the invertibility of rational circulant matrices with any given size. These sufficient conditions consist of linear combinations in terms of the entries in the first row with integer coefficients. Using these conditions we show the invertibility of some family of circulant matrices with particular forms of integers generated by a primitive element in $\mathbb{Z}_p$. Also\, the invertibility of circulant 0\, 1-matrices can be argued combinatorially by applying sufficient conditions. This is joint work with Youngmi Hur.
URL:https://dimag.ibs.re.kr/event/2021-06-29/
LOCATION:Room B232\, IBS (기초과학연구원)
CATEGORIES:Discrete Math Seminar
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