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Lower bounds for small diagonal ramsey numbers
AbstractLet p = 4r + 1 be a prime. Let G be the graph on the p points 0, 1,…, p−1 formed by connecting two points with an edge iff their difference is a quadratic residue mod p. Let k be the size of the largest clique contained in G. Then it is well known that the diagonal Ramsey number R2(k + 1) > p. We show R2(k + 2) > 2p + 2. We also compute k for all p < 3000
Constructive Lower Bounds on Classical Multicolor Ramsey Numbers
This paper studies lower bounds for classical multicolor Ramsey numbers, first by giving a short overview of past results, and then by presenting several general constructions establishing new lower bounds for many diagonal and off-diagonal multicolor Ramsey numbers. In particular, we improve several lower bounds for R_k(4) and R_k(5) for some small k, including 415 \u3c = R_3(5), 634 \u3c = R_4(4), 2721 \u3c = R_4(5), 3416 \u3c = R_5(4) and 26082 \u3c = R_5(5). Most of the new lower bounds are consequences of general constructions
Generalized Paley graphs and their complete subgraphs of orders three and four
Let be an integer. Let be a prime power such that if is even, or, if is odd. The
generalized Paley graph of order , , is the graph with vertex set
where is an edge if and only if is a -th power
residue. We provide a formula, in terms of finite field hypergeometric
functions, for the number of complete subgraphs of order four contained in
, , which holds for all . This generalizes
the results of Evans, Pulham and Sheehan on the original (=2) Paley graph.
We also provide a formula, in terms of Jacobi sums, for the number of complete
subgraphs of order three contained in , . In
both cases we give explicit determinations of these formulae for small . We
show that zero values of (resp.
) yield lower bounds for the multicolor diagonal Ramsey
numbers (resp. ). We state explicitly these
lower bounds for small and compare to known bounds. We also examine the
relationship between both and ,
when is prime, and Fourier coefficients of modular forms
On the proof complexity of Paris-harrington and off-diagonal ramsey tautologies
We study the proof complexity of Paris-Harrington’s Large Ramsey Theorem for bi-colorings of graphs and
of off-diagonal Ramsey’s Theorem. For Paris-Harrington, we prove a non-trivial conditional lower bound
in Resolution and a non-trivial upper bound in bounded-depth Frege. The lower bound is conditional on a
(very reasonable) hardness assumption for a weak (quasi-polynomial) Pigeonhole principle in RES(2). We
show that under such an assumption, there is no refutation of the Paris-Harrington formulas of size quasipolynomial
in the number of propositional variables. The proof technique for the lower bound extends the
idea of using a combinatorial principle to blow up a counterexample for another combinatorial principle
beyond the threshold of inconsistency. A strong link with the proof complexity of an unbalanced off-diagonal
Ramsey principle is established. This is obtained by adapting some constructions due to Erdos and Mills. ˝
We prove a non-trivial Resolution lower bound for a family of such off-diagonal Ramsey principles
Hypergraph Ramsey numbers
The Ramsey number r_k(s,n) is the minimum N such that every red-blue coloring
of the k-tuples of an N-element set contains either a red set of size s or a
blue set of size n, where a set is called red (blue) if all k-tuples from this
set are red (blue). In this paper we obtain new estimates for several basic
hypergraph Ramsey problems. We give a new upper bound for r_k(s,n) for k \geq 3
and s fixed. In particular, we show that r_3(s,n) \leq 2^{n^{s-2}\log n}, which
improves by a factor of n^{s-2}/ polylog n the exponent of the previous upper
bound of Erdos and Rado from 1952. We also obtain a new lower bound for these
numbers, showing that there are constants c_1,c_2>0 such that r_3(s,n) \geq
2^{c_1 sn \log (n/s)} for all 4 \leq s \leq c_2n. When s is a constant, it
gives the first superexponential lower bound for r_3(s,n), answering an open
question posed by Erdos and Hajnal in 1972. Next, we consider the 3-color
Ramsey number r_3(n,n,n), which is the minimum N such that every 3-coloring of
the triples of an N-element set contains a monochromatic set of size n.
Improving another old result of Erdos and Hajnal, we show that r_3(n,n,n) \geq
2^{n^{c \log n}}. Finally, we make some progress on related hypergraph
Ramsey-type problems
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