Seminars
20.03.2024 16:15 Gábor Tardos Alfréd Rényi Institute of Mathematics |
Theoretical computer science Forbidden acyclic patterns in 0-1 matrices |
A zero-one matrix M is said to contain another zero-one matrix A if we can delete some rows and columns of M and replace some 1-entries with 0-entries such that the resulting matrix is A. The extremal function of A, denoted ex(n,A), is the maximum number of 1-entries that an n×n zero-one matrix can have without containing A. The systematic study of this function for various patterns A goes back to the work of Füredi and Hajnal from 1992. The field has many connections to other areas such as classical Turán type extremal graph theory and Davenport-Schinzel theory and has many applications in mathematics and theoretical computer science. The problem has been particularly extensively studied for so-called acyclic matrices. Füredi and Hajnal conjectured an O(n·log n) bound for them, while Pach and Tardos conjectured a weaker n·polylog n bound. Pettie refuted the stronger conjecture with an acyclic pattern whose extremal function he showed to be Ω(n·log n · loglog n).
In a recent joint work with Seth Pettie we found the extremal function ex(n,Ak) asymptotically for certain simple 2×k acyclic patterns to be Θ(n·(log n/loglog n)k−2) for k>3. This shows that the Pach-Tardos conjecture is tight if true. The conjecture itself is still wide open. |
27.03.2024 16:15 Clément Rambaud Université Côte d'Azur |
Theoretical computer science Inversions in oriented graphs |
The inversion of a set X of vertices in an oriented graph consists in reversing the direction of all arcs of the subdigraph induced by X. This generalization of single arc reversal introduced by Belkhechine et al. yields a notion of distance between orientations of a same graph where two orientations are at distance one if and only if there is a set X whose inversion transforms one into the other. First we will discuss the minimum number of inversions required to make an oriented graph acyclic, and in particular a proof of the existence of n-vertex oriented graphs at distance n-o(n) of any acyclic orientation. We also investigate the minimum number of inversions needed to make an oriented graph k-strongly-connected, especially in the case of tournaments. Finally, we show various bounds on the maximum distance between two orientations of a same graph. This gives an undirected graph parameter that we show to be tied to several known parameters, including the star chromatic number and acyclic chromatic number. We also prove that two orientations of a same planar graph are at distance at most 12. Most of these results rely on an algebraic point of view that allows us to use linear algebra over the field with two elements. This is joint work with Guillaume Aubian, Julien Duron, Frédéric Havet, Florian Horsch, Felix Klingelhoefer, Nicolas Nisse, and Quentin Vermande. |
03.04.2024 16:15 David Conlon California Institute of Technology |
Theoretical computer science TBA - 2024.04.03 - David Conlon |
10.04.2024 16:15 Jean Cardinal Université Libre de Bruxelles |
Theoretical computer science TBA - 2024.04.10 - Jean Cardinal |
17.04.2024 16:15 TBA |
Theoretical computer science TBA - 2024.04.17 |
24.04.2024 16:15 Alexander Wolff Universität Würzburg |
Theoretical computer science TBA - 2024.04.24 - Alexander Wolff |
08.05.2024 16:15 TBA |
Theoretical computer science TBA - 2024.05.08 |
15.05.2024 16:15 Marta Kwiatkowska University of Oxford |
Theoretical computer science TBA - 2024.05.15 - Marta Kwiatkowska |
22.05.2024 16:15 TBA |
Theoretical computer science TBA - 2024.05.22 |
29.05.2024 16:15 TBA |
Theoretical computer science TBA - 2024.05.29 |
05.06.2024 16:15 Maria Chudnovsky Princeton University |
Theoretical computer science TBA - 2024.06.05 - Maria Chudnovsky |
12.06.2024 16:15 TBA |
Theoretical computer science TBA - 2024.06.12 |
02.10.2024 16:15 TBA |
Theoretical computer science TBA - 2024.10.02 |
09.10.2024 16:15 TBA |
Theoretical computer science TBA - 2024.10.09 |
Poprzednie referaty
06.03.2024 16:15 Jacob Fox Stanford University |
Theoretical computer science Structure theorems for intersection patterns of geometric objects |
In this talk we discuss Szemerédi-type structure theorems, Ramsey-type theorems, and Turán-type theorems for intersection patterns of geometric objects and their relationships to each other. In particular, we discuss recent such results on intersection graphs of pseudo-segments and an application which gives a new upper bound on the number of edges of a simple topological graph with no k pairwise disjoint edges. Joint work with János Pach and Andrew Suk. |
25.01.2024 17:30 Filip Jasionowicz |
Combinatorial Optimization Four Pages Are Indeed Necessary for Planar Graphs |
An embedding of a graph in a book consists of a linear order of its vertices along the spine of the book and of an assignment of its edges to the pages of the book, so that no two edges on the same page cross. The book thickness of a graph is the minimum number of pages over all its book embeddings. Accordingly, the book thickness of a class of graphs is the maximum book thickness over all its members. In this paper, we address a long-standing open problem regarding the exact book thickness of the class of planar graphs, which previously was known to be either three or four. We settle this problem by demonstrating planar graphs that require four pages in any of their book embeddings, thus establishing that the book thickness of the class of planar graphs is four.
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25.01.2024 16:45 Artemy Oueiski |
Combinatorial Optimization A simple linear time algorithm for cograph recognition |
Cographs are precisely the P4-free graphs. It is shown that a cograph can be uniquely represented by a special tree, called a cotree, where the leaves of the cotree correspond to the vertices of the cograph. An algorithm for recognizing cographs is considered, operating in linear time through two steps. In the first step partition refinement is used to create a factorizing permutation. At the second step, the permutation is tested to verify whether the graph is a cograph. Then algorithms for deriving the characteristics (pathwidth, treewidth, number of cliques) of a cograph from its cotree are explored.
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25.01.2024 16:00 Katzper Michno |
Combinatorial Optimization Another approach to non-repetitive colorings of graphs of bounded degree |
A non-repetitive graph coloring (of vertices or edges) is a coloring such that all sequences of colors induced by paths in the graph are non-repetitive (square-free), which means that they do not contain any consecutive subsequence that is a square. The non-repetitive number of a graph is the minimal number of colors in a non-repetitive vertex coloring (resp. non-repetitive index for coloring edges). There are also list counterparts of these numbers. Many maximal degree related upper bounds for non-repetitive number (resp. index) have been established commonly using the Lovász Local Lemma or entropy compression method. The author of this paper introduces another method of proving these bounds, which is closely related to the entropy compression method, but generates simpler and more elementary proofs. The author provides some minor improvements to non-repetitive number in several cases and matches some of already known bounds using the new technique. |
24.01.2024 16:15 Sergio Cabello University of Ljubljana and IMFM, Slovenia |
Theoretical computer science Packing d-dimensional balls into a (d+1)-dimensional container |
We consider the problems of finding in d+1 dimensions a minimum-volume axis-parallel box, a minimum-volume arbitrarily-oriented box and a minimum-volume convex body into which a given set of d-dimensional unit-radius balls can be packed under translations. We give a constant-factor approximation algorithm for each of these containers. We also show that for n such balls, a container of volume O(n1−1/d) is always sufficient and sometimes necessary. Joint work with Helmut Alt, Otfried Cheong, Ji-won Park and Nadja Seiferth. |
18.01.2024 16:45 Milana Kananovich |
Combinatorial Optimization A Linear Recognition Algorithm for Cographs. A Simple Linear Time LexBFS Cograph Recognition Algorithm. |
Cographs are the graphs formed from a single vertex under the closure of the operations of union and complement. Another characterization of cographs is that they are undirected graphs with no induced paths on four vertices. Cographs have a unique tree representation called a cotree. We consider two linear time algorithms for recognizing cographs and constructing their cotree representation (or the reason why it is not a cograph, the 2nd algorithm gives us P4): a step-by-step recognition algorithm (where we have a list of conditions that must not be violated for the cograph) and LexBFS recognition algorithm (it uses a LexBFS approach, and introduces a new variant of LexBFS, operating on the complement of the given graph G and breaking ties concerning an initial LexBFS).
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18.01.2024 16:00 Sebastian Spyrzewski |
Combinatorial Optimization List coloring with requests |
In this paper we consider the problem of L-coloring graph G with the given list assignment L, but with additional requests giving the preferred color of some vertices. We ask a question of how many of these preferences can be respected while L-coloring G. We present a connection between weighted and unweighted requests and show that for degenerate graphs there is always a constant fraction of preferences that can be satisfied. |
17.01.2024 16:15 Jim Geelen University of Waterloo |
Theoretical computer science Average plane size |
Consider a finite set of distinct points in d-dimensional Euclidean space. A line is said to be spanned if it contains two distinct points from the given set, and a plane is spanned if it contains three non-collinear points from the given set. In 1941, Melchior proved that the average number of given points on a spanned line is bounded above by 3, unless the given points all lie on a single line. We prove that the average number of given points on a spanned plane is bounded above by an absolute constant, unless all of the given points lie on a single plane or they lie on the union of two lines. This is joint work with Rutger Campbell and Matthew Kroeker. |
11.01.2024 16:45 Aleksander Katan |
Combinatorial Optimization Countable graphs are majority 3-choosable |
A majority coloring of a graph is a vertex coloring in which for each vertex there are at least as many bichromatic edges containing that vertex as monochromatic ones. The Unfriendly Partition Conjecture states that every countable graph admits a majority 2-coloring. It is known that for every (not necessarily countable) graph a majority 3-coloring always exists. Anholcer, Bosek, and Grytczuk have recently proven that every countable graph is majority 4-choosable, and we will see an improvement of this result to lists of size 3, as well as a simplified version of the proof that countable graphs are 3-colorable. |
11.01.2024 16:00 Łukasz Gniecki |
Combinatorial Optimization The Alon Tarsi Number of Planar Graphs - a Simple Proof |
The Alon-Tarsi number of a Graph, AT(G), is a value defined by considering eulerian subsets of edges of a chosen orientation of the graph. It has many connections to the domain of graph coloring. For example, the choice number of a graph, ch(G), is bounded by the Alon-Tarsi number, AT(G). In this paper, we will see a simple proof, in the style of Thomassen's induction, of the statement that for any planar graph G, AT(G) is at most 5. Alongside, we will see that any planar G has a matching M, such that AT(G - M) is at most 4. |
10.01.2024 16:15 Peter Allen London School of Economics and Political Science |
Theoretical computer science Universality for degenerate graphs |
A graph G is universal for a family ℱ of graphs if for each F in ℱ there is a copy of F in G (not necessarily induced, and the copies are not necessarily disjoint). Alon and Capalbo considered the case that ℱ is the family of n-vertex graphs with maximum degree k, and showed that there is a universal graph for this family with O(n2-2/k) edges, which is sharp. Alon asked what the answer is if one replaces 'maximum degree' with 'degeneracy'. We give a probabilistic construction of a universal graph for the family of n-vertex d-degenerate graphs with Õ(n2-1/d) edges, which is optimal up to the polylog. In this talk, I will describe the construction and give most of the details of the proof of its universality. This is joint work with Julia Boettcher and Anita Liebenau. |
21.12.2023 16:45 Kamil Galewski |
Combinatorial Optimization On two generalizations of the Alon–Tarsi polynomial method |
The Alon-Tarsi number of a graph G=([n], E) is the smallest integer k, such that there exists a monomial x1d1x2d2...xndn in the expansion of the graph polynomial of G having non-zero coefficient and satisfying di < k for all i∈[n]. Using Combinatorial Nullstellensatz, one can show that this number is an upper bound on the choice number of the graph (and thus on its chromatic number). Alon and Tarsi presented a way of checking non-zeroness of the coefficient of the monomial x1d1...xndn in case di = outdegD(i) for some orientation D of graph G - it is sufficient to check whether the difference between the number of the odd and even Eulerian suborientations of D is non-zero. In this presentation, I will show a generalization of this result - for each D, there exists an infinite family of functions f mapping suborientations of D to real numbers, such that the coefficient mentioned above is non-zero iff the sum of f(A) over all the suborientations A of D is non-zero. |
21.12.2023 16:00 Ignacy Buczek |
Combinatorial Optimization A note on computer-assisted proofs in flag algebras |
With the help of CSDP solvers, one can use computer assistance to obtain correct proofs in flag algebras. In the most common implementations, the programs return the best possible bound on the objective function, together with some information on the extremal graphon. However, for more complicated graphons, this information is usually insufficient to fully retrieve the extremal graphon. We present how one can gather more information on the extremal graphon by adding temporary assumptions to the program, using a non-trivial example that we stumbled upon in our unpublished work on balanced bipartitions of K4-free graphs. |
20.12.2023 16:15 Paweł Rzążewski Warsaw University of Technology |
Theoretical computer science Understanding graphs with no long claws |
A classic result of Alekseev asserts that for connected H the MAX INDEPENDENT SET (MIS) problem in H-free graphs in NP-hard unless H is a path or a subdivided claw. Recently we have witnessed some great progress in understanding the complexity of MIS in Pt-free graphs. The situation for forbidden subdivided claws is, however, much less understood. During the talk we will present some recent advances in understanding the structure of graphs with no long induced claws. We are able to use them to obtain a quasipolynomial-time algorithm for the problem. |
14.12.2023 16:00 Jędrzej Hodor |
Combinatorial Optimization Wythoff's game |
Consider an n×m chessboard with a single queen placed somewhere. There are two players and in order to win, one has to place the queen in the left-bottom corner. A player can either move the queen diagonally towards the left-bottom or vertically towards the left or bottom. It turns out that sometimes the first player has a winning strategy and sometimes the second player. The characterization is mathematically beautiful. The first player has a winning strategy if and only if there is a non-negative integer n such that the queen starts in the position (⌊nφ⌋, ⌊nφ2⌋), where φ is the golden ratio. |
13.12.2023 00:00 |
Theoretical computer science The 17th workshop on Computational Logic and Applications |
07.12.2023 16:00 Piotr Kaliciak |
Combinatorial Optimization Hat guessing numbers of strongly degenerate graphs |
Consider a game with n players, each placed on one of the vertices of graph G. Each player is given a hat, but they cannot see their hat color; they can only see the colors of the hats worn by their neighbors in G. The objective for the players is to ensure that at least one player correctly guesses the color of their hat. The hat guessing number of graph G, denoted by HG(G), is the maximum number of colors for which the players possess a winning strategy. We present an upper bound for the hat guessing number of d-degenerate and outerplanar graphs. |
06.12.2023 16:15 Paul Bastide LaBRI, Bordeaux |
Theoretical computer science Skipless chain decompositions and improved poset saturation bounds |
We show that given m disjoint chains in the Boolean lattice, we can create m disjoint skipless chains that cover the same elements (where we call a chain skipless if any two consecutive elements differ in size by exactly one). By using this result we are able to answer two conjectures about the asymptotics of induced saturation numbers for the antichain, which are defined as follows. For positive integers k and n, a family F of subsets of {1,...,n} is k-antichain saturated if it does not contain an antichain of size k (as induced subposet), but adding any set to F creates an antichain of size k. We use sat*(n,k) to denote the smallest size of such a family. With more work we pinpoint the exact value of sat*(n,k), for all k and sufficiently large n. Previously, exact values for sat*(n,k) were only known for k up to 6. We also show that for any poset P, its induced saturation number (which is defined similar as for the antichain) grows at most polynomially: sat*(n,P)=O(nc), where c≤|P|2/4+1. This is based on joint works with Carla Groenland, Maria-Romina Ivan, Hugo Jacob and Tom Johnston. |
30.11.2023 16:45 Jan Klimczak |
Combinatorial Optimization On the equitable distribution of points on the circle |
The stick-breaking problem is equivalent to the online resource allocation problem, where we possess one unit of resource and we want to fairly distribute fractions of it between people, whose number is unknown at the beginning and upon person's arrival we are only allowed to decrease the share of resource of one person and transfer it to the newcomer. We present various solutions to this problem and analyze their efficiency. |
30.11.2023 16:00 Rafał Pyzik |
Combinatorial Optimization Online Algorithms for Maximum Cardinality Matching with Edge Arrivals |
In the edge arrival model for the online maximum matching problem, edges are sequentially presented and each of them is accepted for the final matching or discarded. We present the Min-Index framework - a family of randomized algorithms for this problem. Using this framework, we show a 5/9-competitive algorithm when the graph is a tree. Moreover, we show that any algorithm in the edge arrival model is at most 0.5914 competitive. |
29.11.2023 16:15 Matthieu Rosenfeld LIRMM, Montpellier |
Theoretical computer science A simple counting argument applied to graph colorings |
The Lovász Local Lemma is one of the central tools of Erdős' probabilistic method. This rather simple lemma has been applied to SAT formulas, graph colorings, hypergraph coloring, combinatorics on words, geometry, and countless other topics. This Lemma essentially tells that given a set of "bad events", under the right conditions, the probability that no events occur is nonzero. It implies the existence of a coloring or an affection of the variables with the desired properties. There are many versions of the Lovász Local Lemma that apply to different situations. Many related techniques that apply to similar problems have emerged in the last 20 years (entropy compression, cluster expansion, local cut lemma...). Recently, I have introduced a counting argument that belongs to the same family of technique. The main interest of this counting argument is that it is really simple to use and it has already been applied to different problems by a few different authors. |
23.11.2023 16:45 Izabela Tylek |
Combinatorial Optimization Any 7-chromatic graph has a K7 or K4,4 as a minor |
One of perhaps the most important and interesting unsolved problems in the field of graph theory is the Hadwiger conjecture, which states that every k-chromatic graph has a Kk-minor. It has been proven to be true for k≤6; the cases k=5 and k=6 have been shown to be equivalent to the four-color theorem. The conjecture remains unsolved for k≥7, but some partial results are known. We will look closer at one of them, showing that any 7-chromatic graph has a K7 or K4,4 as a minor. |
23.11.2023 16:00 Justyna Jaworska |
Combinatorial Optimization An O(n√n) algorithm to color proper circular arcs |
A proper circular arc family F is a set of arcs on a circle such that no arc is contained within another. We consider incidence graphs for such arc families. On proper circular-arc graphs, the coloring problem is polynomially solvable, most recently, in O(n1.5 log n) time (Teng and Tucker). It's due to the fact that the (q-colorability) problem can be reduced to a shortest path problem. In this note, we improve Teng and Tucker’s algorithm obtaining O(n1.5) running time. |
22.11.2023 16:15 Gábor Damásdi Alfréd Rényi Institute of Mathematics |
Theoretical computer science Monochromatic configurations on the circle |
In this lecture we will discuss a surprising combinatorial conjecture. For k≥3 call a k-tuple (d1,d2,...,dk) with d1≥d2≥...≥dk>0 and d1+d2+...+dk=1 a Ramsey k-tuple if the following is true: in every two-colouring of the circle of unit perimeter, there is a monochromatic k-tuple of points in which the distances of cyclically consecutive points, measured along the arcs, are d1,d2,...,dk in some order. By a conjecture of Stromquist, if di=2k-i/(2k-1), then d1,d2,...,dk is Ramsey. Using Sat solvers we showed that the conjecture holds for k≤7. Our main result is a proof of the converse of this conjecture. That is, we show that if (d1,d2,...,dk) is Ramsey, then di=2k-i/(2k-1). We do this by finding connections of the problem to certain questions from number theory about partitioning N into so-called Beatty sequences.
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16.11.2023 16:45 Maciej Sanocki |
Combinatorial Optimization Two-sided Online Bipartite Matching and Vertex Cover: Beating the Greedy Algorithm |
In the original setting of online bipartite matching, vertices from only one side of the bipartite graph are online. This time however we will focus on generalization, in which all vertices can be online. An algorithm for it should maintain a b-matching and try to maximize its size. We show that this problem can be attacked by considering the complementary “dual” problem, a two-sided online bipartite vertex cover. |
16.11.2023 16:00 Katarzyna Kępińska |
Combinatorial Optimization On Two problems of Defective Choosability of Graphs |
Graph G is (k,d,p)-choosable if given list assignment L where |L(v)| is at least k for each vertex v and the number of all available colors is p, there exists L-coloring such that maximum degree of monochromatic subgraph is at most d. This paper shows two constructions of graphs: 1-defective 3-choosable that are not 4-choosable and (k,d,l)-choosable that are not (k,d,l+1)-choosable. |
15.11.2023 16:15 Piotr Micek Jagiellonian |
Theoretical computer science Tight bound for the Erdős-Pósa property of tree minors |
Let T be a tree on t vertices. We prove that for every positive integer k and every graph G, either G contains k pairwise vertex-disjoint subgraphs each having a T minor, or there exists a set X of at most t(k-1) vertices of G such that G-X has no T minor. The bound on the size of X is best possible and improves on an earlier f(t)k bound proved by Fiorini, Joret, and Wood (2013) with some very fast growing function f(t). Our proof is moreover very short and simple. Joint work with Vida Dujmović, Gwenaël Joret, and Pat Morin |
09.11.2023 16:00 Karolina Okrasa Warsaw University of Technology |
Combinatorial Optimization Graph Homomorphisms: From Structure to Algorithms |
For two graphs G and H, a homomorphism from G to H is a function that maps the vertices of G to the vertices of H in a way that edges are preserved. Graph homomorphisms are a generalization of graph colorings: if H is a complete graph on k vertices, then homomorphisms from G to H are precisely the k-colorings of G and vice versa. It seems natural to follow the lines of research for the coloring problem to study the more general homomorphism problem. In the talk, I will focus on determining the complexity of the homomorphism problem (and its list variant) when we assume the class of input instances is somehow restricted, e.g., by bounding some structural parameter of an instance, or excluding the instances that contain some fixed graph as an induced subgraph. We examine to which extent the variety of tools developed to work on coloring problems can be applied, and show more general methods to approach these problems. |
08.11.2023 16:15 Torsten Ueckerdt Karlsruhe Institute of Technology |
Theoretical computer science When Surrounding is not Catching in Cops and Robber |
After a short introduction of the classical game of Cops and Robber on graphs, we shall discuss two recently introduced variants in which the robber only loses when he is completely surrounded by the cops. In the first variant the robber is surrounded when he sits at a vertex v and there is at least one cop on each neighbor of v. In the second variant cops occupy edges of the graph and the robber (still moving on vertices) is surrounded if he sits at a vertex v and there is at least one cop on each incident edge at v. We shall compare these games with each other and also with the classical game in which the robber is already caught when one cop sits on the same vertex as the robber. |
26.10.2023 16:45 Agata Margas |
Combinatorial Optimization Making the Rules of Sports Fairer |
The rules of many sports are not fair - they do not ensure that equally skilled competitors have the same probability of winning. As an example, the penalty shootout in soccer, wherein a coin toss determines which team kicks first on all five penalty kicks, gives a substantial advantage to the first-kicking team, both in theory and in practice. We show that a so-called Catch-Up Rule for determining the order of kicking would not only make the shootout fairer but is also essentially strategyproof. By contrast, the so-called Standard Rule now used for the tiebreaker in tennis is fair. |
26.10.2023 16:00 Mikołaj Kot |
Combinatorial Optimization Circle graphs and monadic second-order logic |
Circle graph is intersection graph of set of chords od a circle. Such set is called chord diagram. It can also be described by word with two occurrences of each letter. If given graph is prime for the split decomposition, it has unique representation as chord diagram, and this diagram can be defined by monadic second-order formulas with even cardinality set predicate. The article also states that a set of circle graphs has bounded clique-width if and only if all the associated chord diagrams have bounded tree-width. |
26.10.2023 14:15 Jan Klimczak, Szymon Wojtulewicz |
Algorytmika Approximating Knapsack and Partition via Dense Subset Sums |
Kwestia złożoności (1 - ε)-aproksymacji problemu plecakowego i problemu podziału pozostaje nierozstrzygnięta. Prezentujemy algorytmy: - (1 - ε)-aproksymacja problemu plecakowego w złożoności O(n + (1/ε)^(2.2)) - (1 - ε)-aproksymacja problemu podziału w złożoności O(n + (1/ε)^(1.25)) Obie techniki wykorzystują poprzednie rezultaty na temat konwolucji gęstych zbiorów. Wykorzystane zostały też nowe sposoby przyspieszenia metody 'dziel i zwyciężaj', która jest często wykorzystywana w problemach addytywnych. |
25.10.2023 16:15 Torsten Mütze University of Warwick |
Theoretical computer science A book proof of the middle levels theorem |
In this lecture I present an elementary and fully self-contained proof of the middle levels conjecture (now theorem), which asserts that the subgraph of the (2n+1)-dimensional hypercube induced by all bitstrings with Hamming weight n or n+1 admits a Hamilton cycle for every n≥1. |
19.10.2023 16:45 Maksym Zub |
Combinatorial Optimization A note concerning the Grundy and b-chromatic number of graphs |
The Grundy number Γ(G) is the maximum number of colors used by the First-Fit coloring of G denoted by Γ(G). Similarly, the b- chromatic number b(G) of G expresses the worst-case behavior of another well-known coloring procedure i.e. color-dominating coloring of G. We obtain some families of graphs F for which there exists a function f(x) such that Γ(G) ≤ f(b(G)), for each graph G from the family. Call any such family (Γ,b)-bounded family. We conjecture that the family of b-monotone graphs is (Γ, b)-bounded and validate the conjecture for some families of graphs. |
19.10.2023 16:00 Jakub Fedak |
Combinatorial Optimization The complexity of coloring circular arcs and chords |
Numerous graph problems, known to be NP-complete, become polynomial when restricted to specific graph types, such as planar graphs or comparability graphs. The article shows the NP-completeness of graph coloring for circular-arc graphs and circle graphs, as well as a polynomial algorithm for producing a K-coloring for circular-arc graphs if one exists. To prove the NP-completeness of graph coloring, we use a polynomial reduction from another NP-complete problem known as the word problem for products of symmetric groups (WPPSG). |
18.10.2023 16:15 Marcelo Campos University of Oxford |
Theoretical computer science An exponential improvement for diagonal Ramsey |
The Ramsey number R(k) is the minimum n such that every red-blue colouring of the edges of the complete graph Kn on n vertices contains a monochromatic copy of Kk. We prove that R(k)≤3.99k. This is the first exponential improvement over the upper bound of Erdős and Szekeres, proved in 1935.
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12.10.2023 16:00 Bartłomiej Błoniarz |
Combinatorial Optimization (Some of) the many uses of Eulerian graphs in graph theory (plus some applications) |
The article showcases diverse associations between Eulerian graphs and other attributes of graphs such as being Hamiltonian, nowhere-zero flows, the cycle-plus-triangles problem, and issues emanating from it. It shows the application of compatible cycle decompositions in creating loopless 4-regular graphs with exactly one Hamiltonian cycle, or in establishing the equivalence between the Chinese Postman Problem and the Planar Bridgeless Minimum Cycle Covering Problem. |
12.10.2023 14:15 Kacper Topolski, Jakub Wąs |
Algorytmika Simple and Faster Algorithms for Knapsack |
Na tym seminarium zdefiniujemy problem plecakowy oraz jego wariacje - wersję 0-1, ograniczoną oraz DiffKnapsack. Przybliżymy najnowsze rezultaty związane z tym problemem. W szczególności zaprezentujemy prosty algorytm randomizowany rozwiązujący dyskretny wariant problemu plecakowego oraz oparty na nim algorytm rozwiązujący wersję ograniczoną. Jest on rozwinięciem pierwszego algorytmu o liniowej zależności względem liczby elementów, zaprezentowanego m.in. przez Adama Polaka. |
12.10.2023 14:15 Kacper Topolski, Jakub Wąs |
Algorytmika Simple and Faster Algorithms for Knapsack |
Na tym seminarium zdefiniujemy problem plecakowy oraz jego wariacje - wersję 0-1, ograniczoną oraz DiffKnapsack. Przybliżymy najnowsze rezultaty związane z tym problemem. W szczególności zaprezentujemy prosty algorytm randomizowany rozwiązujący dyskretny wariant problemu plecakowego oraz oparty na nim algorytm rozwiązujący wersję ograniczoną. Jest on rozwinięciem pierwszego algorytmu o liniowej zależności względem liczby elementów, zaprezentowanego m.in. przez Adama Polaka. |
11.10.2023 16:15 Krzysztof Potępa Jagiellonian University |
Theoretical computer science Better Diameter Algorithms for Bounded VC-dimension Graphs and Geometric Intersection Graphs |
We develop a framework for algorithms finding diameter in graphs of bounded distance Vapnik-Chervonenkis dimension, in (parameterized) sub-quadratic time complexity. The class of bounded distance VC-dimension graphs is wide, including, e.g. all minor-free graphs. We build on the work of Ducoffe et al., improving their technique. With our approach the algorithms become simpler and faster, working in Õ(k·V1-1/d·E) time complexity, where k is the diameter, d is the VC-dimension. Furthermore, it allows us to use the technique in more general setting. In particular, we use this framework for geometric intersection graphs, i.e. graphs where vertices are identical geometric objects on a plane and the adjacency is defined by intersection. Applying our approach for these graphs, we answer a question posed by Bringmann et al., finding a Õ(n7/4) parameterized diameter algorithm for unit square intersection graph of size n, as well as a more general algorithm for convex polygon intersection graphs. This is joint work with Lech Duraj and Filip Konieczny. |
05.10.2023 16:00 Bartłomiej Bosek |
Combinatorial Optimization Some open problems from combinatorics and algorithmics |
The first presented problem concerns the majority coloring of graphs in the undirected and directed cases. A surprising connection with the problem of spreading epidemics in graphs will be shown. The second presented problem concerns the hat guessing game. The most classic results as well as the most interesting unresolved hypotheses will be shown. The last presented problem will concern randomized online algorithms for finding matching in bipartite graphs. Classic algorithms and research directions worth pursuing will be presented. |
04.10.2023 16:15 Avi Wigderson Institute for Advanced Study, Princeton |
Theoretical computer science The Value of Errors in Proofs |
Recently, a group of theoretical computer scientists posted a paper on the Arxiv with the strange-looking title "MIP* = RE", surprising and impacting not only complexity theory but also some areas of math and physics. Specifically, it resolved, in the negative, the "Connes' embedding conjecture" in the area of von-Neumann algebras, and the "Tsirelson problem" in quantum information theory. It further connects Turing's seminal 1936 paper which defined algorithms to Einstein's 1935 paper with Podolsky and Rosen which challenged quantum mechanics. You can find the paper here https://arxiv.org/abs/2001.043 |
15.06.2023 16:45 Julia Biały |
Combinatorial Optimization A game generalizing Hall's Theorem |
Authors investigate starting positions in a particular two-player game, considering scenarios where the first player can have a winning strategy. This work offers a broader interpretation of Hall's Theorem using Vizing's Theorem on edge-coloring in a specialized setting. |
15.06.2023 16:00 Łukasz Selwa |
Combinatorial Optimization Orientations of Graphs with Prescribed Weighted Out-Degrees |
We study the complexity of the orientation problem where the out-neighborhood of every vertex is bounded by some function. This problem can be used to apply Galvin’s kernel method to show that graph G satisfies a certain coloring property. We show that the problem is NP-complete in the case of graphs that are bipartite, planar, and of maximum degree at most 3. We also prove some results on the (f,g)-choosability problem for weighted graphs, including a generalization of Brooks's theorem for weighted graphs. |
14.06.2023 16:15 Fabrizio Frati Università Roma Tre |
Theoretical computer science Currents Trends and Hot Problems in Graph Drawing |
In this expository talk, I will discuss the topics that have attracted the most attention in the graph drawing community in recent years. The talk will try to convey the direction where the research in graph drawing is going, with a focus on the most intriguing open problems in the field. |
07.06.2023 16:15 Michał Seweryn Université Libre de Bruxelles |
Theoretical computer science Recent results in graph product structure theory |
Graph product structure theory describes complicated graphs as subgraphs of strong products of simpler building blocks. Usually, the strong product involves three graphs: a graph of bounded treewidth, a small complete graph, and a path. For example, Dujmović et al. showed that every planar graph is a subgraph of the strong product of a treewidth-3 graph, a complete graph on three vertices, and a path. This theorem has been the key to solving many long-standing problems about planar graphs, and is arguably the most important result of the graph product structure theory. In my talk I will discuss some of the recent results in this field. First I will discuss two generalizations of the product structure theorem for planar graphs to k-planar graphs and k-powers of planar graphs with bounded degree. The distinguishing property of these theorems is that the bound on the treewidth in the product is an absolute constant independent of k and the maximum degree. Then, I will discuss some product structure theorems, where an n-vertex graph is a subgraph of the strong product of two graphs: a graph of constant treewidth, and a complete graph on O(√n) vertices. These theorems are qualitative strengthenings of √n-separator theorems by Lipton-Tarjan and Alon-Seymour-Thomas. Joint works with Marc Distel, Vida Dujmović, David Eppstein, Robert Hickingbotham, Gwenaël Joret, Piotr Micek, Pat Morin, and David Wood |
31.05.2023 16:15 Ola Svensson École Polytechnique Fédérale de Lausanne |
Theoretical computer science The Price of Explainability for Clustering |
Given a set of points in d-dimensional space, an explainable clustering is one where the clusters are specified by a tree of axis-aligned threshold cuts. Dasgupta et al. (ICML 2020) posed the question of the price of explainability: the worst-case ratio between the cost of the best explainable clusterings to that of the best clusterings.
We show that the price of explainability for k-medians is at most 1+Hk−1; in fact, we show that the popular Random Thresholds algorithm has exactly this price of explainability, matching the known lower bound constructions. We complement our tight analysis of this particular algorithm by constructing instances where the price of explainability (using any algorithm) is at least (1−o(1))·ln k, showing that our result is best possible, up to lower-order terms. We also improve the price of explainability for the k-means problem to O(k·lnln k) from the previous O(k·ln k), considerably closing the gap to the lower bounds of Ω(k).
Finally, we study the algorithmic question of finding the best explainable clustering: We show that explainable k-medians and k-means cannot be approximated better than O(ln k), under standard complexity-theoretic conjectures. This essentially settles the approximability of explainable k-medians and leaves open the intriguing possibility to get significantly better approximation algorithms for k-means than its price of explainability.
This is joint work with Anupam Gupta, Madhusudhan Reddy Pittu, and Rachel Yuan |
25.05.2023 16:45 Katarzyna Król |
Combinatorial Optimization Ball Packings and Lorentzian Discrete Geometry |
The problem of packing balls is to find an arrangement of spheres in space so that no spheres overlap. It is popular in the literature to consider packing disks - i.e. two-dimensional spheres. A tangency graph is a graph whose vertices are balls and whose edge is between vertices u and v if ball u and ball v touch each other. We study ball packings whose tangency graph is a higher dimensional grid graph. We give a loose bound on the size of such grid graphs that admit a ball packing. |
25.05.2023 16:00 Jędrzej Kula |
Combinatorial Optimization Playing cards with Vizing's demon |
The paper's authors present a parametrized version of the solitaire game. In this version, players play against a demon whose task is to rearrange cards after each move in a way that the player will not be able to win the game. By defining a specific demon strategy and finding the winning strategy against it, one may prove König and Vizing theorems. |
24.05.2023 16:15 Csaba Tóth California State University, Northridge |
Theoretical computer science Optimal spanners in Euclidean spaces |
For a set S of n points in a metric space (X,d) and a parameter t>1, a t-spanner is a weighted graph G=(S,E) such that the shortest path distance in G approximates the pairwise distances in the metric space up to a factor of at most t (stretch factor). This talk focuses on the d-dimensional Euclidean space in the regime where t is close to 1. Recent research uncovered tight trade-offs for two important quality measures for t-spanners: the sparsity |E(G)|/n and the lightness w(G)/w(MST(S)). We present an algorithm that constructs a t-spanner for a given set of n points in Euclidean d-space, by sparsifying classical Yao-graphs, that attains a worst-case optimal bound on the weight of the spanner. In the online model, a sequence of points arrive one-by-one, and we need to maintain a t-spanner for the first n points for all n. By combining sparse Yao-graphs and hierarchical clustering, we obtain an online algorithm that maintains a light spanner and achieves logarithmic competitive ratio compared to the offline optimum. |
18.05.2023 16:45 Krzysztof Barański |
Combinatorial Optimization A note on degree-constrained subgraphs |
Last semester I presented a paper “A note on polynomials and f-factors of graphs” by Hamed Shirazi and Jacques Verstraëte, who proved two f-factor theorems using the Combinatorial Nullstellensatz. In this work, authors take a closer look at the same theorems and prove them in a different way. |
18.05.2023 16:00 Filip Konieczny |
Combinatorial Optimization On constructive methods in the theory of colour-critical graphs |
k-critical graph is not (k-1)-colorable but every proper subgraph is. The authors take a constructive approach to the theory of critical graphs and show methods of how such graphs can be constructed, composed, and augmented, additionally discussing the advantages and drawbacks of these methods. |
17.05.2023 16:15 John Iacono Université Libre de Bruxelles |
Theoretical computer science The pursuit of the dynamic optimality conjecture via the geometry of binary search trees |
In 1983, Sleator and Tarjan introduced the splay tree, a self-adjusting binary search tree algorithm. Splay trees were conjectured to perform within a constant factor as any offline rotation-based search tree algorithm on every sufficiently long sequence — any binary search tree algorithm that has this property is said to be dynamically optimal. However, currently neither splay trees nor any other tree algorithm is known to be dynamically optimal. In doing so we will present the geometric view of binary search trees, introduced in 2009, where we (with Erik D. Demaine, Dion Harmon, Daniel M. Kane and Mihai Pătraşcu) showed an equivalence between binary search tree algorithms and a simple combinatorial property of points in the plane. Almost all recent progress, which we will survey, towards the forty-year-old dynamic optimality conjecture since then has used this view, as it greatly simplifies reasoning about binary search trees. |
11.05.2023 16:45 Rafał Pyzik |
Combinatorial Optimization Improved lower bounds on the number of edges in list critical and online list critical graphs |
A graph G is k-critical if it is not (k-1)-colorable, but every proper subgraph of G is. Authors improve the bound by Kostochka and Stiebitz for a number of edges in k-critical graphs. The same bound holds for k-list-critical and online k-list-critical graphs improving the bound established by Riasat and Schauz. This result follows from analyzing Alon-Tarsi orientable induced subgraphs satisfying certain conditions.
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11.05.2023 16:00 Aleksander Katan |
Combinatorial Optimization A not 3-choosable planar graph without 3-cycles |
An L-list coloring of graph G is a proper vertex coloring in which every vertex receives a color from a prescribed list L(v). G is said to be k-choosable, if all lists L(v) have cardinality k, and G is L-colorable for any assignment of those lists. The author presents a planar graph without 3-cycles that is not 3-choosable. We will also discuss other topics related to list colorings, such as the fact that every planar graph is 5-choosable.
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10.05.2023 16:15 Clément Rambaud École Normale Supérieure, PSL Paris |
Theoretical computer science Neighborhood complexity of planar graphs |
In a class of graphs of bounded expansion, for every graph in the class, for every non-empty set A of vertices, for every radius r, the number of distinct intersections between A and a ball of radius r is at most f(r)·|A| for some function f depending only on the considered class [Reidl, Sánchez Villaamil and Stravopoulos, 2019]. The function f coming from existing proofs is typically exponential. We prove that in the special case of planar graphs, f can be taken to be a polynomial, and more precisely in O(r4). We also show that a polynomial bound holds more generally for every proper minor-closed class of graphs. This is joint work with Gwenaël Joret. |
04.05.2023 16:45 Rafał Kilar |
Combinatorial Optimization On the structure of k-connected graphs without K_k-minor |
The famous Hadwiger's Conjecture states that every k-chromatic graph must contain the clique Kk as a minor. It remains unproven for k>6. Motivated by this conjecture we can ask about the structure of k-connected graphs without Kk as a minor. We show that any such graph can't have three (k-2)-cliques that share at most three vertices, which is a generalization of previous results in the area. |
04.05.2023 16:00 Bartłomiej Błoniarz |
Combinatorial Optimization Pólya's Permanent Problem |
The permanent of a square matrix is a function very similar to the determinant. It has important applications in counting problems, but computing it is a #P-complete problem. In 1913, Pólya proposed a method to calculate permanents using determinants, which was soon proven to be faulty in certain cases. This led to the question of when Pólya's method can be used, known as Pólya's Permanent Problem. The article provides an overview of the problem, including equivalent versions and a solution to one of the formulations. |
27.04.2023 16:45 Demian Banakh |
Combinatorial Optimization Flip distance to a non-crossing perfect matching |
A non-crossing perfect matching is Euclidean matching on 2n points so that no 2 segments cross. Given some crossing matching, we can iteratively apply the flip operation (fix any 2 crossing segments, and swap the endpoints so that they don't cross) to eventually arrive at a non-crossing matching. We will investigate the upper and lower bounds for the number of flips sufficient and necessary to eliminate all crossings. It is conjectured that θ(n2) flips are always sufficient.
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27.04.2023 16:00 Szymon Salabura |
Combinatorial Optimization Edge lower bounds for list critical graphs, via discharging |
We say that a graph G is k-choosable if G has a proper coloring from every list assignment L with |L(v)|=k for every vertex v. A graph G is k-list-critical if it's not k-choosable, but every proper subgraph of G is. The problem of bounding the number of edges from below in such graphs has been widely studied, starting with the work of Gallai. The authors present a rephrased version of his proof using the discharging method and improve the original result by presenting additional properties of such graphs, enabling a different set of discharging rules. |
27.04.2023 14:15 Justyna Jaworska, Jakub Siuta |
Algorytmika Simple, deterministic, fast (but weak) approximations to edit distance and Dyck edit distance |
Dla problemów znjadowania odległości edycyjnej i odległości edycyjnej Dycka chcemy znaleść szybkie, deterministyczne i proste aproksymacje, z być może dużym współczynnikiem aproksymacji. Dla klasycznej odległości edycyjnej wprowadzimy klasę szybkich i prostych algorytmów nazywanych "algorytmami pojdedycznego skanowania". Saha, w 2014. roku, podał randomizowany algorytm z tej klasy osiągający aproksymację O(d) dla słow x, y takich że ich ogległość edycyjna jest rzędu O(d). W tej pracy prezentujemy: (1) deterministyczny algorytm z wymienionej klasy osiągający podobne rezultaty oraz (2) dowodzimy, że nie istnieje (nawet randomizowany) algorytm z tej klasy, który dawałby lepszą aproksymację. Dla odległości edycyjnej Dycka, Saha zaproponował randomizowaną redukcję z odległości edycyjnej Dycka do klasycznej odległości edycyjnej o koszcie O(log d), gdzie d to odległość edycyjna Dycka. Podamy redukcję deterministyczną której zarówno sfromułowanie jak i udowodnienie poprawności jest prostsze. |
26.04.2023 16:15 David Eppstein University of California, Irvine |
Theoretical computer science The Complexity of Iterated Reversible Computation |
Reversible computation has been studied for over 60 years as a way to evade fundamental physical limits on the power needed for irreversible computational steps, and because quantum computing circuits are necessarily reversible. We study a class of problems based on computing the iterated values of a reversible function. The story leads through Thomason's lollipop algorithm in graph theory, circuit complexity, and reversible cellular automata, to card shuffling, the reflections of light in jewels, and curves on topological surfaces, and involves both PSPACE-hard problems and problems with unexpected polynomial-time algorithms. |
20.04.2023 16:45 Piotr Kaliciak |
Combinatorial Optimization Decomposing 4-connected planar triangulations into two trees and one path |
A graph is 4-connected if no matter which 4 vertices we remove from it, it remains connected. We can decompose every 4-connected planar triangulation into a Hamiltonian path and two trees. Moreover, we can decompose any Hamiltonian planar triangulation into two trees and one spanning tree of degree at most 3. These results are best-possible, this means that we cannot decrease the maximum degree of the first tree. |
20.04.2023 16:00 Kamil Galewski |
Combinatorial Optimization On the discrepancy of circular sequences of reals |
The discrepancy is a function that measures the irregularity of the distribution of a given sequence of real numbers. The authors present a new method to measure discrepancy for sequences of reals lying on a circle of circumference 1, as a more sensitive alternative to the previously known functions. They also show a tight upper bound for this function. |
19.04.2023 16:15 Pat Morin Carleton University |
Theoretical computer science Proof of the Clustered Hadwiger Conjecture |
Hadwiger's Conjecture asserts that every Kh-minor-free graph is properly (h-1)-colourable. We prove the following improper analogue of Hadwiger's Conjecture: for fixed h, every Kh-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. It 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≥s, we show that every Ks,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. This joint work with Vida Dujmović, Louis Esperet, and David R. Wood. |
13.04.2023 16:45 Łukasz Gniecki |
Combinatorial Optimization Sequences of points on a circle |
Consider a sequence of points a1, a2, a3, ... on a circle with radius 1/(2π), in other words, numbers mod 1. The numbers a1, a2, ..., an define n intervals with a total length of 1. Denote by M[n,r](a) and m[n,r](a) the largest and the smallest length of consecutive r intervals. We can think of how the values n·M[n, r](a) and n·m[n, r](a) will behave if we go with n to infinity. In particular, for a given sequence a we can find the upper limit of n·M: L[r](a) = limsup n·M[n,r](a) and the lower limit of n·m: l[r](a) = liminf n·m[n,r](a). We can go further and consider the greatest lower bound on L[r](a) (g.l.b. in short) and the lowest upper bound on l[r](a) (l.u.b. in short), overall sequences a. The authors derive bounds on this g.l.b. and l.u.b. and in the case of r = 1, they prove these bounds are tight by giving an example of a sequence a which satisfies these bounds. |
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