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ABSTRACT A theorem of Shearer states that every ‐vertex triangle‐free graph of maximum degree contains an independent set of size at least . Ajtai, Komlós, Pintz, Spencer and Szemerédi proved that every ‐uniform ‐vertex “uncrowded” hypergraph of maximum degree has an independent set of size at least for some depending only on . Shearer asked whether his method for triangle‐free graphs could be extended to uniform hypergraphs. In this paper, we answer this in the affirmative, thereby giving a short proof of the theorem of Ajtai, Komlós, Pintz, Spencer and Szemerédi for a wider class of “locally sparse” hypergraphs.more » « less
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Abstract For an integer , the Erdős–Rogers function is the maximum integer such that every ‐vertex ‐free graph has a ‐free induced subgraph with vertices. It is known that for all , as . In this paper, we show that for all , there exists a constant such thatThis improves previous bounds of order by Dudek, Retter and Rödl and answers a question of Warnke.more » « less
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ABSTRACT For ak‐uniform hypergraph and a positive integer , the Ramsey number denotes the minimum such that every ‐vertex ‐free ‐uniform hypergraph contains an independent set of vertices. A hypergraph isslowly growingif there is an ordering of its edges such that for each . We prove that if is fixed and is any non‐k‐partite slowly growing ‐uniform hypergraph, then for ,In particular, we deduce that the off‐diagonal Ramsey number is of order , where is the triple system . This is the only 3‐uniform Berge triangle for which the polynomial power of its off‐diagonal Ramsey number was not previously known. Our constructions use pseudorandom graphs and hypergraph containers.more » « less
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Abstract Building on recent work of Mattheus and Verstraëte, we establish a general connection between Ramsey numbers of the form for a fixed graph and a variant of the Zarankiewicz problem asking for the maximum number of 1s in an by ‐matrix that does not have any matrix from a fixed finite family derived from as a submatrix. As an application, we give new lower bounds for the Ramsey numbers and , namely, and . We also show how the truth of a plausible conjecture about Zarankiewicz numbers would allow an approximate determination of for any fixed integer .more » « less
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A natural open problem in Ramsey theory is to determine those -graphs for which the off-diagonal Ramsey number grows polynomially with . We make substantial progress on this question by showing that if is tightly connected or has at most two tight components, then grows polynomially if and only if is contained in an iterated blowup of an edge.more » « less
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