Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2018/iii/paper-130/1/ii/solution
Past exam of the mathematics course of the University of Cambridge 2018 iii Paper 130 1 ii Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-05
Let be an intersecting family. For a word , put . Give the red color if contains some member of , and the blue color otherwise.
Consider any combinatorial line with active set . At variable value , , so that word is red. At variable value , . No member of can be contained in this , because every member meets . Thus that word is blue. No such line is monochromatic, provingIf is empty there is no permitted active set at all, so it is also not an adequate family of active coordinate sets.
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