Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2019/iii/paper-109/1/ii/solution

Let be an intersecting family, with . By the Iterated local LYM inequality, its upper shadow in level satisfies
The family of complements also lies in level and has cardinality . It is disjoint from the upper shadow: if for , then , contradicting intersection. Both families fit inside the th level, so
Thus replacing the Kruskal-Katona theorem by Local LYM gives only
This agrees with the Erdős-Ko-Rado theorem when but is weaker when .

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