
arXiv: 2310.01394
An ordered $r$-matching of size $n$ is an $r$-uniform hypergraph on a linearly ordered set of vertices, consisting of $n$ pairwise disjoint edges. Two ordered $r$-matchings are isomorphic if there is an order-preserving isomorphism between them. A pair of twins in an ordered $r$-matching is formed by two vertex disjoint isomorphic sub-matchings. Let $t^{(r)}(n)$ denote the maximum size of twins one may find in every ordered $r$-matching of size $n$. By relating the problem to that of largest twins in permutations and applying some recent Erdős-Szekeres-type results for ordered matchings, we show that $t^{(r)}(n)=Ω\left(n^{\frac{3}{5\cdot(2^{r-1}-1)}}\right)$ for every fixed $r\geqslant 2$. On the other hand, $t^{(r)}(n)=O\left(n^{\frac{2}{r+1}}\right)$, by a simple probabilistic argument. As our main result, we prove that, for almost all ordered $r$-matchings of size $n$, the size of the largest twins achieves this bound.
hypergraphs, Edge subsets with special properties (factorization, matching, partitioning, covering and packing, etc.), QA1-939, FOS: Mathematics, Mathematics - Combinatorics, twins, Combinatorics (math.CO), Hypergraphs, ordered matchings, Mathematics
hypergraphs, Edge subsets with special properties (factorization, matching, partitioning, covering and packing, etc.), QA1-939, FOS: Mathematics, Mathematics - Combinatorics, twins, Combinatorics (math.CO), Hypergraphs, ordered matchings, Mathematics
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