Combinatorics · Extremal set · Doubling chains · Odd part decomposition
Let \(S = \{1, 2, \ldots, 20\}\). What is the largest possible number of elements of a subset \(A \subseteq S\) with the property that \(2x \notin A\) whenever \(x \in A\)?
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Serbian Municipal Competition 2002, high school grade I, category A, problem 4. Organized by the Mathematical Society of Serbia (DMS). Source