UNIFORM TURAN DENSITY OF CYCLES

In the early 1980s, Erdos and Sos initiated the study of the classical Turan problem with a uniformity condition: the uniform Turan density of a hypergraph H is the infimum over all d for which any sufficiently large hypergraph with the property that all its linear-size subhypergraphs have density at least d contains H. In particular, they raise the questions of determining the uniform Turan densities of K-4((3)-) and K-4((3)). The former question was solved only recently by Glebov, Kral', and Volec [Israel J. Math. 211 (2016), pp. 349-366] and Reiher, Rodl, and Schacht [J. Eur. Math. Soc. 20 (2018), pp. 1139-1159], while the latter still remains open for almost 40 years. In addition to K-4((3)-), the only 3-uniform hypergraphs whose uniform Turan density is known are those with zero uniform Turan density classified by Reiher, Rodl and Schacht [J. London Math. Soc. 97 (2018), pp. 77-97] and a specific family with uniform Turan density equal to 1/27. We develop new tools for embedding hypergraphs in host hypergraphs with positive uniform density and apply them to completely determine the uniform Turan density of a fundamental family of 3-uniform hypergraphs, namely tight cycles C-l((3)). The uniform Turan density of C-l((3)), l >= 5, is equal to 4/27 if l is not divisible by three, and is equal to zero otherwise. The case l = 5 resolves a problem suggested by Reiher.