An Approximate Version of the Tree Packing Conjecture via Random Embeddings

We prove that for any pair of constants a>0 and D and for n sufficiently large, every family of trees of orders at most n, maximum degrees at most D, and with at most n(n-1)/2 edges in total packs into the complete graph of order (1+a)n. This implies asymptotic versions of the Tree Packing Conjecture of Gyarfas from 1976 and a tree packing conjecture of Ringel from 1963 for trees with bounded maximum degree. A novel random tree embedding process combined with the nibble method forms the core of the proof

An approximate version of the Loebl-Komlós-Sós conjecture

Loebl, Komlós, and Sós conjecture that if at least half of the vertices of a graph G have degree at least some k ∈ N, then every tree with at most k edges is a subgraph of G. Our main result is an approximate version of this conjecture for large enough n = |V (G)|, and k linear in n. We extend our result to a slightly larger class of subgraphs. Namely, we show that G contains as subgraphs all bipartite connected graphs of order k + 1 with at most k + c edges, where c is some constant in n. Also, we derive from our result an asymptotic bound for the Ramsey number of trees. We prove that r(Tk, Tm) ≤ k + m + o(k + m), provided that lim inf(k/m),lim inf(m/k)> 0

Counting patterns in strings and graphs

We study problems related to finding and counting patterns in strings and graphs. In the string-regime, we are interested in counting how many substring of a text are at Hamming (or edit) distance at most to a pattern . Among others, we are interested in the fully-compressed setting, where both and are given in a compressed representation. For both distance measures, we give the first algorithm that runs in (almost) linear time in the size of the compressed representations. We obtain the algorithms by new and tight structural insights into the solution structure of the problems. In the graph-regime, we study problems related to counting homomorphisms between graphs. In particular, we study the parameterized complexity of the problem #IndSub(), where we are to count all -vertex induced subgraphs of a graph that satisfy the property . Based on a theory of Lovász, Curticapean et al., we express #IndSub() as a linear combination of graph homomorphism numbers to obtain #W[1]-hardness and almost tight conditional lower bounds for properties that are monotone or that depend only on the number of edges of a graph. Thereby, we prove a conjecture by Jerrum and Meeks. In addition, we investigate the parameterized complexity of the problem #Hom(ℋ → ) for graph classes ℋ and . In particular, we show that for any problem in the class #W[1], there are classes ℋ_ and _ such that is equivalent to #Hom(ℋ_ → _ ).Wir untersuchen Probleme im Zusammenhang mit dem Finden und Zählen von Mustern in Strings und Graphen. Im Stringbereich ist die Aufgabe, alle Teilstrings eines Strings zu bestimmen, die eine Hamming- (oder Editier-)Distanz von höchstens zu einem Pattern haben. Unter anderem sind wir am voll-komprimierten Setting interessiert, in dem sowohl , als auch in komprimierter Form gegeben sind. Für beide Abstandsbegriffe entwickeln wir die ersten Algorithmen mit einer (fast) linearen Laufzeit in der Größe der komprimierten Darstellungen. Die Algorithmen nutzen neue strukturelle Einsichten in die Lösungsstruktur der Probleme. Im Graphenbereich betrachten wir Probleme im Zusammenhang mit dem Zählen von Homomorphismen zwischen Graphen. Im Besonderen betrachten wir das Problem #IndSub(), bei dem alle induzierten Subgraphen mit Knoten zu zählen sind, die die Eigenschaft haben. Basierend auf einer Theorie von Lovász, Curticapean, Dell, and Marx drücken wir #IndSub() als Linearkombination von Homomorphismen-Zahlen aus um #W[1]-Härte und fast scharfe konditionale untere Laufzeitschranken zu erhalten für , die monoton sind oder nur auf der Kantenanzahl der Graphen basieren. Somit beweisen wir eine Vermutung von Jerrum and Meeks. Weiterhin beschäftigen wir uns mit der Komplexität des Problems #Hom(ℋ → ) für Graphklassen ℋ und . Im Besonderen zeigen wir, dass es für jedes Problem in #W[1] Graphklassen ℋ_ und _ gibt, sodass äquivalent zu #Hom(ℋ_ → _ ) ist

Sufficient conditions for embedding trees

We study sufficient degree conditions that force a host graph to contain a given class of trees. This setting involves some well-known problems from the area of extremal graph theory. The most famous one is the Erdős-Sós conjecture that asserts that every graph with average degree greater than k − 1 contains any tree on k + 1 vertices. Our two main results are the following. We prove an approximate version of the Erdős-Sós conjecture for dense graphs and trees with sublinear max- imum degree. We also study a natural refinement of the Loebl-Komlós-Sós conjecture and prove it is approximately true for dense graphs. Both results are based on the so-called regularity method. The second mentioned result is a joint work with T. Klimošová and D. Piguet.

Postačující podmínky pro vnořování stromů

Studujeme podmínky na stupně vrcholů, které vynucují, že daný graf obsahuje libovolný strom z dané třídy. Tento typ problémů zahrnuje některé známé problémy z oblasti extremální teorie grafů. Nejslavnějším z nich je domněnka Erdős-Sósové, která tvrdí, že každý graf s průměrným stupněm vyšším než k − 1 obsahuje libovolný strom na k + 1 vrcholech. Naše dva hlavní výsledky jsou následující. Dokazujeme přibližnou verzi domněnky Erdős-Sósové pro husté grafy a stromy se sublineárním maximál- ním stupněm. Dále studujeme přirozené zobecnění domněnky Loebl-Komlós- Sósové a opět dokážeme přibližnou verzi této domněnky pro husté grafy. Oba výsledky jsou založeny na takzvané regularity metodě. Druhý výsledek je společnou prací s T. Klimošovou a D. Piguet. 1We study sufficient degree conditions that force a host graph to contain a given class of trees. This setting involves some well-known problems from the area of extremal graph theory. The most famous one is the Erdős-Sós conjecture that asserts that every graph with average degree greater than k − 1 contains any tree on k + 1 vertices. Our two main results are the following. We prove an approximate version of the Erdős-Sós conjecture for dense graphs and trees with sublinear max- imum degree. We also study a natural refinement of the Loebl-Komlós-Sós conjecture and prove it is approximately true for dense graphs. Both results are based on the so-called regularity method. The second mentioned result is a joint work with T. Klimošová and D. Piguet. 1Katedra aplikované matematikyDepartment of Applied MathematicsMatematicko-fyzikální fakultaFaculty of Mathematics and Physic

LIPIcs, Volume 261, ICALP 2023, Complete Volume

LIPIcs, Volume 261, ICALP 2023, Complete Volum