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New bounds of two hypergraph Ramsey problems
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New bounds of two hypergraph Ramsey problems
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We focus on two hypergraph Ramsey problems. First, we consider the Erd\H{o}s-Hajnal function $r_k(k+1,t;n)$. In 1972, Erd\H{o}s and Hajnal conjectured that the tower growth rate of $r_k(k+1,t;n)$ is $t-1$ for each $2\le t\le k$. To finish this conjecture, it remains to show that the tower growth rate of $r_4(5,4;n)$ is three. We prove a superexponential lower bound for $r_4(5,4;n)$, which improves the previous best lower bound $r_4(5,4;n)\geq 2^{\Omega(n^2)}$ from Mubayi and Suk (\emph{J. Eur. Math. Soc., 2020}). Second, we prove an upper bound for the hypergraph Erd\H{o}s-Rogers function $f^{(k)}_{k+1,k+2}(N)$ that is an iterated $(k-3)$-fold logarithm in $N$ for each $k\geq 5$. This improves the previous upper bound that is an iterated $(k-13)$-fold logarithm in $N$ for $k\ge14$ due to Mubayi and Suk (\emph{J. London Math. Soc., 2018}), in which they conjectured that $f^{(k)}_{k+1,k+2}(N)$ is an iterated $(k-2)$-fold logarithm in $N$ for each $k\ge3$.
Forward citations
Cited by 7 Pith papers
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A double-exponential lower bound for $r_4(5,n)$
r_4(5,n) is at least 2^{2^{c n^{1/7}}}, determining the tower growth rate of r_k(k+1,n) for hypergraph Ramsey numbers.
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Hypergraph Erd\H{o}s--Rogers functions with consecutive clique sizes
For every fixed s ≥ 4, f^{(4)}_{s,s+1}(n) = (log n)^{o(1)}, from a new 3-uniform bound f^{(3)}_{s,s+1}(n) = O(log n/log log n).
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Hypergraph Erd\H{o}s--Rogers functions with consecutive clique sizes
For fixed s≥4, every n-vertex K_{s+1}^{(4)}-free 4-graph has a K_s^{(4)}-free set of size (log n)^{o(1)}, via a new O(log n / log log n) bound for 3-graphs.
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New Tower-Type Lower Bounds for Hypergraph Ramsey Numbers
Improves r_k(k+1,k+1) > s_3(⌊k/2⌋-2) for k≥6 and proves s_3(k) ≥ (twr_{k-2}(2))^2 for k≥5, yielding r_k(k+1,k+1) > (twr_{⌊k/2⌋-4}(2))^2 for k≥14.
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A Note on Generalized Erd\H{o}s-Rogers Problems
f^{(4)}_{5^{-},6}(N) equals (log log N) to the Theta(1) power, with improved lower bounds r_4(6,n) >= 2^{2^{c sqrt(n)}} and r_k(k+2,n).
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An improved double-exponential lower bound for $r_4(5,n)$
The Ramsey number r_4(5,n) is at least 2^{2^{Omega(n^{1/5})}}, an improvement over the prior 2^{2^{Omega(n^{1/7})}} achieved by reducing greedy layers in the construction from seven to five.
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An improved double-exponential lower bound for $r_4(5,n)$
The paper establishes the improved lower bound r_4(5,n) >= 2^{2^{Omega(n^{1/5})}} for the 4-uniform 5-clique Ramsey number by reducing greedy local-maxima selection from seven layers to five in a modified construction.
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