Past exam of the mathematics course of the University of Cambridge 2006 iii Paper 12 2 iii Solution Created 2026-10-03 Updated 2026-10-14
False in general. Take , , andIt is a Frankl family with members. Two members each use at least three of the first four points, so they share at least two of those points. Hence it is t-intersecting with , and .
The first fifteen rank-four sets in lexicographic order are precisely those containing . The sixteenth is , which intersects the earlier set only at one. Therefore the lex initial segment of size seventeen is not two-intersecting. This proves that lexicographic replacement can destroy two-intersection; an ordinary intersecting bound cannot justify preservation of higher-order intersection.