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Ander Lamaison

Publications and source records attributed to Ander Lamaison.

At least 19 recordsLinked to original sources
arXiv2026

Solution of uniform Tur\'an's Tetrahedron Problem

Tur\'an's Tetrahedron Problem asks to determine the Tur\'an density of the complete hypergraph $K_4^{(3)}$ (tetrahedron). This problem, posed by Tur\'an in 1941, is one of the most famous problems in extremal combinatorics and its solution would attract \$500 prize from Erd\H{o}s. In the 1980s, Erd\H{o}s and S\'os asked to determine Tur\'an densities of $K_4^{(3)-}$ (broken tetrahedron) and $K_4^{(3)}$ (tetrahedron) when edges are constrained to be uniformly distributed in the host hypergraph. The presumably easier case of the broken tetrahedron was solved by Glebov, Kr\'al' and Volec [Israel J. Math. 211 (2016), 349-366] and Reiher, R\"odl and Schacht [J. Eur. Math. Soc. 20 (2018), 1139-1159]. We solve the tetrahedron case by proving that the uniform Tur\'an density of $K_4^{(3)}$ is equal to 1/2; this confirms that R\"odl's lower bound construction from 1986 is optimal.

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arXiv2026

Logarithmic convergence of finite projective planes

In this paper, we study the so-called log-convergence of graphs defined by Bal\'azs Szegedy (arXiv:1504.00858). We answer his Question 4 affirmatively: the sequence of incidence graphs of projective planes over finite fields log-converges, and the limit coincides with that of a particular random graph model.

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arXiv2026

Fundamental cycles in grid graphs

We show that the average length of a fundamental cycle with respect to any fixed spanning tree of the $n\times n$ square grid is at least $\Omega(\log n)$; the bound is asymptotically tight. This result answers in the affirmative a question posed by McCarty in relation to sparse representations of binary matroids.

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arXiv2026

Ramsey size linear and generalization

More than thirty years ago, Erdős, Faudree, Rousseau, and Schelp posed a fundamental question in extremal graph theory: What is the optimal constant $c_k$ such that $r(C_{2k+1}, G) \le c_k m$ for any graph $G$ with $m$ edges and no isolated vertices? In this paper, we make a significant step towards answering this question by proving that $r(C_{2k+1}, G) \le (2 + o(1)) m + p,$ where $p$ denotes the number of vertices in $G$. Additionally, we extend the work of Goddard and Kleitman and independently Sidorenko, who proved that $r(K_3, G) \le 2m + 1$ for any graph $G$ with $m$ edges and no isolated vertices. We generalize their findings to the clique version, establishing that $r(K_r, G) \le c_r m^{(r-1)/2}$, and to the multicolor setting, showing that $r_{k+1}(K_3; G) \le c_k m^{(k+1)/2}.$

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arXiv2025

Spanning Components and Surfaces Under Minimum Vertex Degree

We study minimum vertex-degree conditions in 3-uniform hypergraphs for (tight) spanning components and (combinatorial) surfaces. Our main results show that a 3-uniform hypergraph $G$ on $n$ vertices contains a spanning component if $δ_1(G) \gtrsim \tfrac{1}{2} \binom{n}{2}$ and a spanning copy of any surface if $δ_1(G) \gtrsim \tfrac{5}{9} \binom{n}{2}$, which in both cases is asymptotically optimal. This extends the work of Georgakopoulos, Haslegrave, Montgomery, and Narayanan who determined the corresponding minimum codegree conditions in this setting.

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arXiv2025

Hypergraphs with uniform Turán density equal to 8/27

In the 1980s, Erdős and Sós initiated the study of Turán problems with a uniformity condition on the distribution of edges: the uniform Turán density of a hypergraph $H$ is the infimum over all $d$ for which any sufficiently large hypergraph with the property that all its linear-size subhypergraphs have density at least $d$ contains $H$. In particular, they asked to determine the uniform Turán densities of $K_4^{(3)-}$ and $K_4^{(3)}$. After more than 30 years, the former was solved in [Israel J. Math. 211 (2016), 349-366] and [J. Eur. Math. Soc. 20 (2018), 1139-1159], while the latter still remains open. Till today, there are known constructions of 3-uniform hypergraphs with uniform Turán density equal to 0, 1/27, 4/27 and 1/4 only. We extend this list by a fifth value: we prove an easy to verify condition for the uniform Turán density to be equal to 8/27 and identify hypergraphs satisfying this condition.

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arXiv2025

Ramsey multiplicity of apices of trees

A graph $H$ is common if its Ramsey multiplicity, i.e., the minimum number of monochromatic copies of $H$ contained in any $2$-edge-coloring of $K_n$, is asymptotically the same as the number of monochromatic copies in the random $2$-edge-coloring of $K_n$. Erdős conjectured that every complete graph is common, which was disproved by Thomason in the 1980s. Till today, a classification of common graphs remains a widely open challenging problem. Grzesik, Lee, Lidický and Volec [Combin. Prob. Comput. 31 (2022), 907--923] conjectured that every $k$-apex of any connected Sidorenko graph is common. We prove for $k\le 5$ that the $k$-apex of any tree is common.

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arXiv2025

Property O and Erdős--Szekeres properties in linear hypergraphs

An oriented $k$-uniform hypergraph, or oriented $k$-graph, is said to satisfy Property O if, for every linear ordering of its vertex set, there is some edge oriented consistently with this order. The minimum number $f(k)$ of edges in a $k$-graph with Property O was first studied by Duffus, Kay, and Rödl, and later improved by Kronenberg, Kusch, Lamaison, Micek, and Tran. In particular, they established the bounds $k! + 1 \le f(k) \le \left(\lfloor\tfrac{k}{2}\rfloor+1 \right) k! - \lfloor\tfrac{k}{2}\rfloor(k-1)!$ for every $k \ge 2$. In this note, we extend the study of Property O to the linear setting. We determine the minimum number $f'(k)$ of edges in a linear $k$-graph up to a $\operatorname{poly}(k)$ multiplicative factor, showing that $\frac{(k!)^2}{2e^2k^4} \le f'(k) \le (1+o(1)) \cdot 4 k^6 \ln^2 k \cdot (k!)^2$. Our approach also yields bounds on the minimum number $n'(k)$ of vertices in an oriented linear $k$-graph with Property O. Additionally, we explore the minimum number of edges and vertices required in a linear $k$-graph satisfying the newly introduced Erdős--Szekeres properties.

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arXiv2025

Uniform Turán density beyond 3-graphs

In the 1980s, Erdős and Sós first introduced an extremal problem on hypergraphs with density constraints. Given an $r$-uniform hypergraph $F$ (or $r$-graph for short), its uniform Turán density $π_u(F)$ is the smallest value of $d$ in which every hypergraph $H$ in which every linear-sized subhypergraph of $H$ has edge density at least $d$ contains $F$ as a subgraph. The first non-zero value of $π_u(F)$ was not found until 30 years later. Progress in studying the set of values of the uniform Turán density of $r$-graphs has been uneven in terms of $r$: to this day there are infinitely many non-zero values known for $r=3$, a single non-zero value known for $r=4$ and none for $r\geq 5$. In this paper we obtain the first explicit values of $π_u$ for all uniformities, by proving that for every $r\geq 3$ there exist $r$-graphs $F$ with $π_u(F)=1/4$ and with $π_u(F)=\binom{r}{2}^{-\binom{r}{2}}$.

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arXiv2025

Closure property of contraction-depth of matroids

Contraction$^*$-depth is a matroid depth parameter analogous to tree-depth of graphs. We establish the matroid analogue of the classical graph theory result asserting that the tree-depth of a graph $G$ is the minimum height of a rooted forest whose closure contains $G$ by proving the following for every matroid $M$ (except the trivial case when $M$ consists of loops and coloops only): the contraction$^*$-depth of $M$ plus one is equal to the minimum contraction-depth of a matroid containing $M$ as a restriction.

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arXiv2025

Uniform Tur\'an density -- palette classification

In the 1980s, Erd\H{o}s and S\'os initiated the study of Tur\'an hypergraph problems with a uniformity condition on the distribution of edges, i.e., determining density thresholds for the existence of a hypergraph H in a host hypergraph with edges uniformly distributed. In particular, Erd\H{o}s and S\'os asked to determine the uniform Tur\'an densities of the hypergraphs $K_4^{(3)-}$ and $K_4^{(3)}$. After more than 30 years, the former was solved by Glebov, Kr\'al' and Volec [Israel J. Math. 211 (2016), 349-366] and Reiher, R\"odl and Schacht [J. Eur. Math. Soc. 20 (2018), 1139-1159], while the latter still remains open. In these two cases and several additional cases, the tight lower bounds are provided by a so-called palette construction. Lamaison [arXiv:2408.09643] has recently showed that the uniform Tur\'an density of a 3-uniform hypergraph H is equal to the supremum of the densities of palettes that H is not colorable with. We give a necessary and sufficient condition, which is easy to verify, on the existence of a 3-uniform hypergraph colorable by a set of palettes and not colorable by another given set of palettes. We also demonstrate how our result can be used to prove the existence of 3-uniform hypergraphs with specific values of the uniform Tur\'an density.

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arXiv2024

Four-coloring Eulerian triangulations of the torus

Hutchinson, Richter and Seymour [J. Combin. Theory Ser. B 84 (2002), 225-239] showed that every Eulerian triangulation of an orientable surface that has a sufficiently high representativity is 4-colorable. We give an explicit bound on the representativity in the case of the torus by proving that every Eulerian triangulation of the torus with representativity at least 10 is 4-colorable. We also observe that the bound on the representativity cannot be decreased to less than 8 as there exists a non-4-colorable Eulerian triangulation of the torus with representativity 7.

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arXiv2024

The uniform Turán density of large stars

We asymptotically resolve the the uniform Turán density problem for the large stars. In particular, we show that the uniform Turán density of the $k$-star $S_k$ is $\frac{k^2-5k+7}{(k-1)^2}$ for $k\ge 48$, matching a lower construction by Reiher, Rödl and Schacht.

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arXiv2024

Palettes determine uniform Turán density

Turán problems, which concern the minimum density threshold required for the existence of a particular substructure, are among the most fundamental problems in extremal combinatorics. We study Turán problems for hypergraphs with an additional uniformity condition on the edge distribution. This kind of Turán problems was introduced by Erdős and Sós in the 1980s but it took more than 30 years until the first non-trivial exact results were obtained when Glebov, Král' and Volec [Israel J. Math. 211 (2016), 349--366] and Reiher, Rödl and Schacht [J. Eur. Math. Soc. 20 (2018), 1139--1159] determined the uniform Turán density of $K_4^{(3)-}$. Subsequent results exploited the powerful hypergraph regularity method, developed by Gowers and by Nagle, Rödl and Schacht about two decades ago. Central to the study of the uniform Turán density of hypergraphs are palette constructions, which were implicitly introduced by Rödl in the 1980s. We prove that palette constructions always yield tight lower bounds, unconditionally confirming present empirical evidence. This results in new and simpler approaches to determining uniform Turán densities, which completely bypass the use of the hypergraph regularity method.

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arXiv2024

On $3$-graphs with vanishing codegree Turán density

For a $k$-uniform hypergraph (or simply $k$-graph) $F$, the codegree Turán density $π_{\mathrm{co}}(F)$ is the supremum over all $α$ such that there exist arbitrarily large $n$-vertex $F$-free $k$-graphs $H$ in which every $(k-1)$-subset of $V(H)$ is contained in at least $αn$ edges. Recently, it was proved that for every $3$-graph $F$, $π_{\mathrm{co}}(F)=0$ implies $π_{\therefore}(F)=0$, where $π_{\therefore}(F)$ is the uniform Turán density of $F$ and is defined as the supremum over all $d$ such that there are infinitely many $F$-free $k$-graphs $H$ satisfying that any induced linear-size subhypergraph of $H$ has edge density at least $d$. In this paper, we introduce a layered structure for $3$-graphs which allows us to obtain the reverse implication: every layered $3$-graph $F$ with $π_{\therefore}(F)=0$ satisfies $π_{\mathrm{co}}(F)=0$. Along the way, we answer in the negative a question of Falgas-Ravry, Pikhurko, Vaughan and Volec [J. London Math. Soc., 2023] about whether $π_{\therefore}(F)\leqπ_{\mathrm{co}}(F)$ always holds. In particular, we construct counterexamples $F$ with positive but arbitrarily small $π_{\mathrm{co}}(F)$ while having $π_{\therefore}(F)\ge 4/27$.

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arXiv2024

The dimension of the region of feasible tournament profiles

Erd\H os, Lovász and Spencer showed in the late 1970s that the dimension of the region of $k$-vertex graph profiles, i.e., the region of feasible densities of $k$-vertex graphs in large graphs, is equal to the number of non-trivial connected graphs with at most $k$ vertices. We determine the dimension of the region of $k$-vertex tournament profiles. Our result, which explores an interesting connection to Lyndon words, yields that the dimension is much larger than just the number of strongly connected tournaments, which would be the answer expected as the analogy to the setting of graphs.

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arXiv2022

Hypergraphs with minimum positive uniform Turán density

Reiher, Rödl and Schacht [J. London Math. Soc. 97 (2018), 77--97] showed that the uniform Turán density of every $3$-uniform hypergraph is either $0$ or at least $1/27$, and asked whether there exist $3$-uniform hypergraphs with uniform Turán density equal or arbitrarily close to $1/27$. We construct $3$-uniform hypergraphs with uniform Turán density equal to $1/27$.

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