Turán problems and shadows. I: Paths and cycles
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Publication:472166
DOI10.1016/J.JCTA.2014.09.005zbMath1302.05129arXiv1308.4120OpenAlexW2059283278WikidataQ105583652 ScholiaQ105583652MaRDI QIDQ472166
Dhruv Mubayi, Alexandr V. Kostochka, Jacques Verstraete
Publication date: 19 November 2014
Published in: Journal of Combinatorial Theory. Series A (Search for Journal in Brave)
Abstract: A $k$-path is a hypergraph P_k = e_1,e_2,...,e_k such that |e_i cap e_j| = 1 if |j - i| = 1 and e_i cap e_j is empty otherwise. A k-cycle is a hypergraph C_k = e_1,e_2,.. ,e_k obtained from a (k-1)-path e_1,e_2,...,e_{k-1} by adding an edge e_k that shares one vertex with e_1, another vertex with e_{k-1} and is disjoint from the other edges. Let ex_r(n,G) be the maximum number of edges in an r-graph with n vertices not containing a given r-graph G. We determine ex_r(n, P_k) and ex_r(n, C_k) exactly for all k ge 4 and r ge 3 and $n$ sufficiently large and also characterize the extremal examples. The case k = 3 was settled by Frankl and F"{u}redi. This work is the next step in a long line of research beginning with conjectures of ErdH os and S'os from the early 1970's. In particular, we extend the work (and settle a recent conjecture) of F"uredi, Jiang and Seiver who solved this problem for P_k when r ge 4 and of F"uredi and Jiang who solved it for C_k when r ge 5. They used the delta system method, while we use a novel approach which involves random sampling from the shadow of an r-graph.
Full work available at URL: https://arxiv.org/abs/1308.4120
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