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A $k$-path is a hypergraph P_k = e_1,e_2,...,e_k such that |e_i \cap e_j| = 1if |j - i| = 1 and e_i \cap e_j is empty otherwise. A k-cycle is a hypergraphC_k = e_1,e_2,.. ,e_k obtained from a (k-1)-path e_1,e_2,...,e_{k-1} by addingan edge e_k that shares one vertex with e_1, another vertex with e_{k-1} and isdisjoint from the other edges.  Let ex_r(n,G) be the maximum number of edges in an r-graph with n verticesnot 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 alsocharacterize the extremal examples. The case k = 3 was settled by Frankl andF\"{u}redi.  This work is the next step in a long line of research beginning withconjectures of Erd\H os and S\'os from the early 1970's. In particular, weextend the work (and settle a recent conjecture) of F\"uredi, Jiang and Seiverwho solved this problem for P_k when r \ge 4 and of F\"uredi and Jiang whosolved it for C_k when r \ge 5. They used the delta system method, while we usea novel approach which involves random sampling from the shadow of an r-graph.

Turan Problems and Shadows I: Paths and Cycles