Size Ramsey number for graphs of bounded degree
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The size Ramsey number \( \hat{r}(G,H)\) is the least integer \( m\) for which there exists a graph \( F\) with \( m\) edges so that in any coloring of the edges of \( F\) in red and blue, there is always either a red copy of \( G\) or a blue copy of \( H\). Sometimes we write \( F \rightarrow (G,H)\) to denote this. For \( G=H\), we denote \( \hat{r}(G,G)\) by \( \hat{r}(G)\).
A size Ramsey problem for bounded degree graphs(proposed by Beck and Erdös [6])
For a graph \(G\) on \(n\) vertices with bounded degree \(d\) , prove that \[ \hat{r}(G) \leq c n \] where \(c\) depends only on \(d\).The case for paths was proved by Beck [2] (see also [6]) by using the following very nice result of Pósa [7]: Suppose that in a graph \( G\), any subset \( X\) of the vertex set of size at most \( n\) satisfies:
Based on this result, Alon and Chung [1] explicitly construct a graph with \( cn\) edges so that no matter how we delete all but an \( \epsilon\)-fraction of the vertices or edges, the remaining graph still contains a path of length \( n\).
We point out that a directed version of this problem was considered by Erdös, Graham and Szemerédi[3] in 1975. Let \( g(n)\) denote the least integer such that there is a directed acyclic graph \( G\) with \( g(n)\) edges having the property that for any set \( X\) of \( n\) vertices of \( G\), there is a directed path on \( G\) of length \( n\) which does not hit \( X\). Then they show
Friedman and Pippenger [4] extended Pósa's result:
Suppose that in a graph \( G\), any subset \( X\) consisting of at most \( 2n-2\) vertices satisfies:
Using the above fact, they showed that
Haxell, Kohayakawa, and Łuczak [5] proved that the size Ramsey number for \( C_n\) has a linear upper bound.
Bibliography | |
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1 | N. Alon and F. R. K. Chung,
Explicit constructions of linear-sized tolerant networks, Discrete Math.
72 (1988), 15-20.
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2 |
J. Beck, On size Ramsey number of paths, trees, and circuits, I,
J. Graph Theory 7 (1983), 115-129.
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3 | P. Erdös, R. L. Graham and E. Szemerédi. On sparse graphs with dense long paths, Computers and mathematics with applications, pp. 365-369, Pergamon, Oxford, 1976
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4 |
J. Friedman and N. Pippenger, Expanding graphs contain all small trees,
Combinatorica 7 (1987), 71-76.
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5 |
P. E. Haxell, Y. Kohayakawa and T. Łuczak, The induced size-Ramsey
number of cycles, Combin. Probab. Comput. 4 (1995), 217-239.
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6 |
J. Nešetril and V. Rödl, eds.,
Mathematics of Ramsey Theory, Springer-Verlag, Berlin, 1990.
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7 |
L. Pósa,
Hamiltonian circuits in random graphs,
Discrete Math. 14 (1976), 359-364.
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