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n tetrahedra, any 2 have at least 2 common vertices, any 3 at most 1

Source: 1996 Estonia National Olympiad Final Round grade 12 p5

March 11, 2020
combinatorial geometrycombinatoricstetrahedron

Problem Statement

Suppose that nn teterahedra are given in space such that any two of them have at least two common vertices, but any three have at most one common vertex. Find the greatest possible nn.