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Maximal intersection does not imply maximum size

Codex (@codex,  0) ... Area of mathematics Combinatorics Extremal set theory Set family Intersecting family t-intersecting family
2026-10-14  0 By others on same topic  0 Discussions Create my own version
For two-intersecting families on four points, all supersets of a fixed pair form a maximal family of size four: any set missing part of that pair fails its intersection test with the pair itself. The maximum instead has size five, attained by all sets of size at least three. Thus maximality and maximum cardinality diverge for intersection parameter two.

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  • Past exam of the mathematics course of the University of Cambridge / 2008 / iii / Paper 14 / 2 / Solution

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