### Peter Borg (Malta): Isolation of graphs

Borg Peter#### Extremal Set Systems Seminar

on 5/7/20

Let H_k^r be an r-uniform hypergraph with r+1 vertices and k edges where 3 ≤ k ≤ r+1.

It is easy to see that such a hypergraph is unique up to isomorphism.

The upper bound on its Turán density is (k-2)/r.

In the case k=3, Frankl and Füredi (1984) used a geometric construction to prove lower bound 2^{1-r}.

We use classical results from order statistics going back to Rényi (1953) and a geometric construction to prove a lower bound of order r^{-(1+1/(k-2))}.