On constants in the Füredi–Hajnal and the Stanley–Wilf conjecture

AbstractFor a given permutation matrix P, let fP(n) be the maximum number of 1-entries in an n×n (0,1)-matrix avoiding P and let SP(n) be the set of all n×n permutation matrices avoiding P. The Füredi–Hajnal conjecture asserts that cP:=limn→∞fP(n)/n is finite, while the Stanley–Wilf conjecture asserts that sP:=limn→∞|SP(n)|n is finite.In 2004, Marcus and Tardos proved the Füredi–Hajnal conjecture, which together with the reduction introduced by Klazar in 2000 proves the Stanley–Wilf conjecture.We focus on the values of the Stanley–Wilf limit (sP) and the Füredi–Hajnal limit (cP). We improve the reduction and obtain sP⩽2.88cP2 which decreases the general upper bound on sP from sP⩽constconstO(klog(k)) to sP⩽constO(klog(k)) for any k×k permutation matrix P. In the opposite direction, we show cP=O(sP4.5).For a lower bound, we present for each k a k×k permutation matrix satisfying cP=Ω(k2)

Maximum size of reverse-free sets of permutations

Two words have a reverse if they have the same pair of distinct letters on the same pair of positions, but in reversed order. A set of words no two of which have a reverse is said to be reverse-free. Let F(n,k) be the maximum size of a reverse-free set of words from [n]^k where no letter repeats within a word. We show the following lower and upper bounds in the case n >= k: F(n,k) \in n^k k^{-k/2 + O(k/log k)}. As a consequence of the lower bound, a set of n-permutations each two having a reverse has size at most n^{n/2 + O(n/log n)}.Comment: 10 page

Ramsey numbers of ordered graphs

An ordered graph is a pair $\mathcal{G}=(G,\prec)$ where $G$ is a graph and $\prec$ is a total ordering of its vertices. The ordered Ramsey number $\overline{R}(\mathcal{G})$ is the minimum number $N$ such that every ordered complete graph with $N$ vertices and with edges colored by two colors contains a monochromatic copy of $\mathcal{G}$. In contrast with the case of unordered graphs, we show that there are arbitrarily large ordered matchings $\mathcal{M}_n$ on $n$ vertices for which $\overline{R}(\mathcal{M}_n)$ is superpolynomial in $n$. This implies that ordered Ramsey numbers of the same graph can grow superpolynomially in the size of the graph in one ordering and remain linear in another ordering. We also prove that the ordered Ramsey number $\overline{R}(\mathcal{G})$ is polynomial in the number of vertices of $\mathcal{G}$ if the bandwidth of $\mathcal{G}$ is constant or if $\mathcal{G}$ is an ordered graph of constant degeneracy and constant interval chromatic number. The first result gives a positive answer to a question of Conlon, Fox, Lee, and Sudakov. For a few special classes of ordered paths, stars or matchings, we give asymptotically tight bounds on their ordered Ramsey numbers. For so-called monotone cycles we compute their ordered Ramsey numbers exactly. This result implies exact formulas for geometric Ramsey numbers of cycles introduced by K\'arolyi, Pach, T\'oth, and Valtr.Comment: 29 pages, 13 figures, to appear in Electronic Journal of Combinatoric

Graph sharing games: complexity and connectivity

We study the following combinatorial game played by two players, Alice and Bob, which generalizes the Pizza game considered by Brown, Winkler and others. Given a connected graph G with nonnegative weights assigned to its vertices, the players alternately take one vertex of G in each turn. The first turn is Alice's. The vertices are to be taken according to one (or both) of the following two rules: (T) the subgraph of G induced by the taken vertices is connected during the whole game, (R) the subgraph of G induced by the remaining vertices is connected during the whole game. We show that if rules (T) and/or (R) are required then for every epsilon > 0 and for every positive integer k there is a k-connected graph G for which Bob has a strategy to obtain (1-epsilon) of the total weight of the vertices. This contrasts with the original Pizza game played on a cycle, where Alice is known to have a strategy to obtain 4/9 of the total weight. We show that the problem of deciding whether Alice has a winning strategy (i.e., a strategy to obtain more than half of the total weight) is PSPACE-complete if condition (R) or both conditions (T) and (R) are required. We also consider a game played on connected graphs (without weights) where the first player who violates condition (T) or (R) loses the game. We show that deciding who has the winning strategy is PSPACE-complete.Comment: 22 pages, 11 figures; updated references, minor stylistical change

On the Geometric Ramsey Number of Outerplanar Graphs

We prove polynomial upper bounds of geometric Ramsey numbers of pathwidth-2 outerplanar triangulations in both convex and general cases. We also prove that the geometric Ramsey numbers of the ladder graph on $2n$ vertices are bounded by $O(n^{3})$ and $O(n^{10})$, in the convex and general case, respectively. We then apply similar methods to prove an $n^{O(\log(n))}$ upper bound on the Ramsey number of a path with $n$ ordered vertices.Comment: 15 pages, 7 figure

Ramseyovské otázky v euklidovském prostoru

One of the problems in Euclidean Ramsey theory is to determine the chromatic number of the Euclidean space. The chromatic number of a space is the minimum number of colors with which the whole space can be colored so that no two points of the same color are at unit distance. We prove that the chromatic number of the six-dimensional real space is at least 11 and that the chromatic number of the seven-dimensional rational space is at least 15. In addition we give a new proof of the lower bound 9 for the chromatic number of the five-dimensional real space. We also simplify the proof of the lower bound 7 for the four-dimensional real space. It is known that the chromatic number of the n-dimensional real space grows exponentially in n. We show some of its subspaces, in which the growth is slower than exponential. We also summarize previous results for normed spaces in general and for some interesting non-Euclidean spaces.Jedním ze základních problémů euklidovské Ramseyovy teorie je určení barevnosti euklidovského prostoru. Barevnost prostoru je nejmenší počet barev, se kterými lze celý prostor obarvit tak, aby žádné dva stejnobarevné body nebyly v jednotkové vzdálenosti. V práci je ukázáno, že barevnost šestirozměrného reálného prostoru je alespoň 11 a že barevnost sedmirozměrného racionálního prostoru je alespoň 15. Dále je předveden nový důkaz dolního odhadu devět pro barevnost pětirozměrného reálného prostoru a zjednodušen důkaz dolního odhadu sedm pro čtyřrozměrný reálný prostor. Je známo, že barevnost n-rozměrného reálného prostoru roste exponenciálně v n. Ukážeme některé podprostory reálného prostoru, pro které barevnost roste pomaleji než exponenciálně. Dále shrneme předchozí výsledky pro obecné normované prostory a některé konkrétní neeuklidovské prostory.Department of Applied MathematicsKatedra aplikované matematikyFaculty of Mathematics and PhysicsMatematicko-fyzikální fakult