For ordinary graphs it is known that any graph G with more edges than the Turan number of Ks must contain several copies of Ks, and a copy of Ks+1-, the complete graph on s+1 vertices with one missing edge. Erdos asked if the same result is true for Ks3, the complete 3-uniform hypergraph on s vertices. In this note, we show that for small values of n, the number of vertices in G, the answer is negative for s=4. For the second property, that of containing a Ks+13-, we show that for s=4 the answer is negative for all large n as well, by proving that the Turan density of K53- is greater than that of K43.
机构:
Nanjing Normal Univ, Sch Math Sci, Nanjing 210023, Jiangsu, Peoples R China
Nanjing Normal Univ, Inst Math, Nanjing 210023, Jiangsu, Peoples R ChinaNanjing Normal Univ, Sch Math Sci, Nanjing 210023, Jiangsu, Peoples R China