Large triangle-free subgraphs in graphs withoutK4
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It is shown that for arbitrary positiveε there exists a graph withoutK4 and so that all its subgraphs containing more than 1/2 +ε portion of its edges contain a triangle (Theorem 2). This solves a problem of Erdös and Nešetřil. On the other hand it is proved that such graphs have necessarily low edge density (Theorem 4).
Theorem 3 which is needed for the proof of Theorem 2 is an analog of Goodman's theorem , it shows that random graphs behave in some respect as sparse complete graphs.
Theorem 5 shows the existence of a graph on less than 1012 vertices, withoutK4 and which is edge-Ramsey for triangles.
KeywordsRandom Graph Complete Graph Edge Density
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