IMO 1979 LL CZS16

Let Q be a square with side length 6. Find the smallest integer

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IMO 1979 LL CZS16

Origin: CZS

Problem

Let Q be a square with side length 6. Find the smallest integer n such that in Q there exists a set S of n points with the property that any square with side 1 completely contained in Q contains in its interior at least one point from S.