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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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X-Robots-Tag:noindex
X-PUBLISHED-TTL:PT1H
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:20210506T100000
DTEND;TZID=Asia/Seoul:20210506T110000
DTSTAMP:20260424T164601
CREATED:20210319T045807Z
LAST-MODIFIED:20240705T190027Z
UID:3813-1620295200-1620298800@dimag.ibs.re.kr
SUMMARY:Raul Lopes\, Adapting the Directed Grid Theorem into an FPT Algorithm
DESCRIPTION:The Grid Theorem of Robertson and Seymour [JCTB\, 1986] is one of the most important tools in the field of structural graph theory\, finding numerous applications in the design of algorithms for undirected graphs. An analogous version of the Grid Theorem in digraphs was conjectured by Johnson et al. [JCTB\, 2001] \, and proved by Kawarabayashi and Kreutzer [STOC\, 2015]. They showed that there is a function f(k) such that every digraph of directed tree-width at least f(k) contains a cylindrical grid of order k as a butterfly minor and stated that their proof can be turned into an XP algorithm\, with parameter k\, that either constructs a decomposition of the appropriate width\, or finds the claimed large cylindrical grid as a butterfly minor. \nIn this talk\, we present the ideas used in our adaptation of the Directed Grid Theorem into an FPT algorithm. We provide two FPT algorithms with parameter k. The first one either produces an arboreal decomposition of width 3k-2 or finds a haven of order k in a digraph D. The second one uses a bramble B that naturally occurs in digraphs of large directed tree-width to find a well-linked set of order k that is contained in the set of vertices of a path hitting all elements of B. As tools to prove these results\, we show how to solve a generalized version of the problem of finding balanced separators for a given set of vertices T in FPT time with parameter |T|. \nJoint work with Victor Campos\, Ana Karolinna Maia\, and Ignasi Sau.
URL:https://dimag.ibs.re.kr/event/2021-05-06/
LOCATION:Zoom ID: 869 4632 6610 (ibsdimag)
CATEGORIES:Virtual Discrete Math Colloquium
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BEGIN:VEVENT
DTSTART;TZID=Asia/Seoul:20210506T161500
DTEND;TZID=Asia/Seoul:20210506T171500
DTSTAMP:20260424T164601
CREATED:20210427T013100Z
LAST-MODIFIED:20240705T185031Z
UID:4010-1620317700-1620321300@dimag.ibs.re.kr
SUMMARY:Hong Liu\, Sublinear expander and embeddings sparse graphs
DESCRIPTION:A notion of sublinear expander has played a central role in the resolutions of a couple of long-standing conjectures in embedding problems in graph theory\, including e.g. the odd cycle problem of Erdős and Hajnal that the harmonic sum of odd cycle length in a graph diverges with its chromatic number. I will survey some of these developments.
URL:https://dimag.ibs.re.kr/event/2021-05-06-2/
LOCATION:Zoom
CATEGORIES:Colloquium
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BEGIN:VEVENT
DTSTART;TZID=Asia/Seoul:20210511T163000
DTEND;TZID=Asia/Seoul:20210511T173000
DTSTAMP:20260424T164601
CREATED:20210420T015716Z
LAST-MODIFIED:20240705T185043Z
UID:3969-1620750600-1620754200@dimag.ibs.re.kr
SUMMARY:Mark Siggers\, The list switch homomorphism problem for signed graphs
DESCRIPTION:A signed graph is a graph in which each edge has a positive or negative sign. Calling two graphs switch equivalent if one can get from one to the other by the iteration of the local action of switching all signs on edges incident to a given vertex\, we say that there is a switch homomorphism from a signed graph $G$ to a signed graph $H$ if there is a sign preserving homomorphism from $G’$ to $H$ for some graph $G’$ that is switch equivalent to $G$.  By reductions to CSP this problem\, and its list version\, are known to be either polynomial time solvable or NP-complete\, depending on $H$.  Recently those signed graphs $H$ for which the switch homomorphism problem is in $P$ were characterised.  Such a characterisation is yet unknown for the list version of the problem. \nWe talk about recent work towards such a characterisation and about how these problems fit in with bigger questions that still remain around the recent CSP dichotomy theorem.
URL:https://dimag.ibs.re.kr/event/2021-05-11/
LOCATION:Room B232\, IBS (기초과학연구원)
CATEGORIES:Discrete Math Seminar
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BEGIN:VEVENT
DTSTART;TZID=Asia/Seoul:20210512T170000
DTEND;TZID=Asia/Seoul:20210512T180000
DTSTAMP:20260424T164601
CREATED:20210319T045925Z
LAST-MODIFIED:20240705T190026Z
UID:3816-1620838800-1620842400@dimag.ibs.re.kr
SUMMARY:Johannes Carmesin\, A Whitney type theorem for surfaces: characterising graphs with locally planar embeddings
DESCRIPTION:Given a graph\, how do we construct a surface so that the graph embeds in that surface in an optimal way? Thomassen showed that for minimum genus as optimality criterion\, this problem would be NP-hard. Instead of minimum genus\, here we use local planarity — and provide a polynomial algorithm. \nOur embedding method is based on Whitney’s trick to use matroids to construct embeddings in the plane. Consequently we obtain a characterisation of the graphs admitting locally planar embeddings in surfaces in terms of a certain matroid being co-graphic.
URL:https://dimag.ibs.re.kr/event/2021-05-12/
LOCATION:Zoom ID: 869 4632 6610 (ibsdimag)
CATEGORIES:Virtual Discrete Math Colloquium
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Asia/Seoul:20210518T163000
DTEND;TZID=Asia/Seoul:20210518T173000
DTSTAMP:20260424T164601
CREATED:20210420T015329Z
LAST-MODIFIED:20240705T185044Z
UID:3967-1621355400-1621359000@dimag.ibs.re.kr
SUMMARY:Pascal Gollin\, Enlarging vertex-flames in countable digraphs
DESCRIPTION:A rooted digraph is a vertex-flame if for every vertex v there is a set of internally disjoint directed paths from the root to v whose set of terminal edges covers all ingoing edges of v. It was shown by Lovász that every finite rooted digraph admits a spanning subdigraph which is a vertex-flame and large\, where the latter means that it preserves the local connectivity to each vertex from the root. A structural generalisation of vertex-flames and largeness to infinite digraphs was given by Joó and the analogue of Lovász’ result for countable digraphs was shown. \nIn this talk\, I present a strengthening of this result stating that in every countable rooted digraph each vertex-flame can be extended to a large vertex-flame. \nJoint work with Joshua Erde and Attila Joó.
URL:https://dimag.ibs.re.kr/event/2021-05-18/
LOCATION:Room B232\, IBS (기초과학연구원)
CATEGORIES:Discrete Math Seminar
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BEGIN:VEVENT
DTSTART;TZID=Asia/Seoul:20210521T170000
DTEND;TZID=Asia/Seoul:20210521T180000
DTSTAMP:20260424T164601
CREATED:20210319T050153Z
LAST-MODIFIED:20240705T190024Z
UID:3818-1621616400-1621620000@dimag.ibs.re.kr
SUMMARY:Benjamin Bumpus\, Directed branch-width: A directed analogue of tree-width
DESCRIPTION:Many problems that are NP-hard in general become tractable on `structurally recursive’ graph classes. For example\, consider classes of bounded tree- or clique-width. Since the 1990s\, many directed analogues of tree-width have been proposed. However\, many natural problems (e.g. directed HamiltonPath and MaxCut) remain intractable on such digraph classes of `bounded width’. \nIn this talk\, I will introduce a new tree-width analogue for digraphs called directed branch-width which allows us to define digraph classes for which many problems (including directed HamiltonPath and MaxCut)  become linear-time solvable. Furthermore\, via the definition of directed branch-width\, I will obtain a generalisation to digraphs of Gurski and Wanke’s characterization of graph classes of bounded tree-width in terms of their line graphs. \nThis is joint work with Kitty Meeks and William Pettersson.
URL:https://dimag.ibs.re.kr/event/2021-05-21/
LOCATION:Zoom ID: 869 4632 6610 (ibsdimag)
CATEGORIES:Virtual Discrete Math Colloquium
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Asia/Seoul:20210525T163000
DTEND;TZID=Asia/Seoul:20210525T173000
DTSTAMP:20260424T164601
CREATED:20210520T102659Z
LAST-MODIFIED:20240707T081345Z
UID:4112-1621960200-1621963800@dimag.ibs.re.kr
SUMMARY:Ben Lund\, Limit shape of lattice Zonotopes
DESCRIPTION:A convex lattice polytope is the convex hull of a set of integral points. Vershik conjectured the existence of a limit shape for random convex lattice polygons\, and three proofs of this conjecture were given in the 1990s by Bárány\, by Vershik\, and by Sinai. To state this old result more precisely\, there is a convex curve $L \subset [0\,1]^2$ such that the following holds. Let $P$ be a convex lattice polygon chosen uniformly at random from the set of convex lattice polygons with vertices in $[0\,N]^2$. Then\, for $N$ sufficiently large\, $(1/N)P$ will be arbitrarily close (in Hausdorff distance) to $L$ with high probability. It is an open question whether there exists a limit shape for three dimensional polyhedra. \nI will discuss this problem and some relatives\, as well as joint work with Bárány and Bureaux on the existence of a limit shape for lattice zonotopes in all dimensions.
URL:https://dimag.ibs.re.kr/event/2021-05-25/
LOCATION:Room B232\, IBS (기초과학연구원)
CATEGORIES:Discrete Math Seminar
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BEGIN:VEVENT
DTSTART;TZID=Asia/Seoul:20210526T170000
DTEND;TZID=Asia/Seoul:20210526T180000
DTSTAMP:20260424T164601
CREATED:20210424T122241Z
LAST-MODIFIED:20240707T081338Z
UID:3986-1622048400-1622052000@dimag.ibs.re.kr
SUMMARY:Dimitrios M. Thilikos\, Bounding Obstructions sets: the cases of apices of minor closed classes
DESCRIPTION:Given a minor-closed graph class ${\cal G}$\, the (minor) obstruction of ${\cal G}$ is the set of all minor-minimal graphs not in ${\cal G}$. Given a non-negative integer $k$\, we define the $k$-apex of ${\cal A}$ as the class containing every graph $G$ with a set $S$ of vertices whose removal from $G$ gives a graph on ${\cal G}$. We prove that every obstruction of the $k$-apex of ${\cal G}$ has size bounded by some 4-fold exponential function of $p(k)$ where p is a polynomial function whose degree depends on the size of the minor-obstructions of ${\cal G}$. This bound drops to a 2-fold exponential one when ${\cal G}$ excludes some apex graph as a minor (i.e.\, a graph in the $1$-apex of planar graphs). \nJoint work with Ignasi Sau and Giannos Stamoulis.
URL:https://dimag.ibs.re.kr/event/2021-05-26/
LOCATION:Zoom ID: 869 4632 6610 (ibsdimag)
CATEGORIES:Virtual Discrete Math Colloquium
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