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X-WR-CALNAME:Discrete Mathematics Group
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:20250101T000000
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BEGIN:VEVENT
DTSTART;TZID=Asia/Seoul:20260901T163000
DTEND;TZID=Asia/Seoul:20260901T173000
DTSTAMP:20260801T015407Z
CREATED:20260801T015407Z
LAST-MODIFIED:20260801T015407Z
UID:13026-1788280200-1788283800@dimag.ibs.re.kr
SUMMARY:Ben Lund\, TBA
DESCRIPTION:
URL:https://dimag.ibs.re.kr/event/2026-09-01/
LOCATION:Room B332\, IBS (기초과학연구원)
CATEGORIES:Discrete Math Seminar
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Asia/Seoul:20260908T163000
DTEND;TZID=Asia/Seoul:20260908T173000
DTSTAMP:20260812T074452Z
CREATED:20260812T064149Z
LAST-MODIFIED:20260812T074452Z
UID:13089-1788885000-1788888600@dimag.ibs.re.kr
SUMMARY:Olga Medrano Martín del Campo\, TBA
DESCRIPTION:
URL:https://dimag.ibs.re.kr/event/2026-09-08/
LOCATION:Room B332\, IBS (기초과학연구원)
CATEGORIES:Discrete Math Seminar
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Asia/Seoul:20260915T163000
DTEND;TZID=Asia/Seoul:20260915T173000
DTSTAMP:20260821T131519Z
CREATED:20260821T131407Z
LAST-MODIFIED:20260821T131519Z
UID:13138-1789489800-1789493400@dimag.ibs.re.kr
SUMMARY:Gabriëlle Zwaneveld\, On Seymour-tight orientations
DESCRIPTION:I discuss ‘almost counterexamples’ to Seymour’s second neighbourhood conjecture. In what we call Seymour-tight orientations\, the size of the first neighbourhood of each vertex equals the size of its second neighbourhood. We give several examples and constructions. Specifically\, we prove that the class of Seymour-tight orientations is closed under taking (generalized) lexicographic products. Moreover\, the lexicographic product of a putative counterexample to Seymour’s second neighbourhood conjecture and a Seymour-tight orientation is again a counterexample. \nUsing lexicographic products\, we show that if the conjecture is false\, then there exist counterexamples that are close to regular tournaments\, and moreover that any digraph occurs as an induced subgraph of a counterexample. We then use this same machinery to construct special putative counterexamples to Sullivan’s conjecture. \nThe inherent symmetry of these orientations give access to an algebraic perspective. Seymour-tight orientations that are also Cayley digraphs correspond to special pairs of critical sets in groups\, which connects potentially to additive combinatorics. We use Kemperman’s theorem to characterize those Seymour-tight orientations that are the Cayley digraph of an abelian group.
URL:https://dimag.ibs.re.kr/event/2026-09-15/
LOCATION:Room B332\, IBS (기초과학연구원)
CATEGORIES:Discrete Math Seminar
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Asia/Seoul:20260922T163000
DTEND;TZID=Asia/Seoul:20260922T173000
DTSTAMP:20260802T034024Z
CREATED:20260717T080520Z
LAST-MODIFIED:20260802T034024Z
UID:12910-1790094600-1790098200@dimag.ibs.re.kr
SUMMARY:David Wood\, Proof of the Clustered Hadwiger Conjecture
DESCRIPTION:Hadwiger famously conjectured that every $K_h$-minor-free graph is properly $(h-1)$-colourable. This talk will present the following improper analogue of Hadwiger’s Conjecture: for fixed $h$\, every $K_h$-minor-free graph is $(h-1)$-colourable with monochromatic components of bounded size. The number of colours is best possible regardless of the size of monochromatic components. This solves an open problem of Edwards\, Kang\, Kim\, Oum and Seymour [SIAM J. Disc. Math. 2015]\, and concludes a line of research initiated in 2007. Similarly\, for fixed $t\geqslant s$\, we show that every $K_{s\,t}$-minor-free graph is $(s+1)$-colourable with monochromatic components of bounded size. The number of colours is best possible\, solving an open problem of van de Heuvel and Wood [J. London Math. Soc. 2018]. We actually prove a single theorem from which both of the above results are immediate corollaries. For an excluded apex minor\, the result is strengthened  as follows: for fixed $t \geqslant s \geqslant 3$\, and for any fixed apex graph $X$\, every $K_{s\,t}$-subgraph-free $X$-minor-free graph is $(s+1)$-colourable with monochromatic components of bounded size. The number of colours is again best possible. This is joint work with Vida Dujmović\, Louis Esperet and Pat Morin [arXiv:2306.06224].
URL:https://dimag.ibs.re.kr/event/2026-09-22/
LOCATION:Room B332\, IBS (기초과학연구원)
CATEGORIES:Discrete Math Seminar
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