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MathNet 2022-part-01 — 197 problems (EN–VI) Question 97

Math Olympiad Vault · MathNet 2022-part-01 — 197 problems (EN–VI)

Question 97

A subset $A$ of the natural numbers $\mathbb{N}=\{0,1,2, \ldots\}$ is called good if every integer $n \gt 0$ has at most one prime divisor $p$ such that $n-p \in A$.

a. Show that the set $S=\{0,1,4,9, \ldots\}$ of perfect squares is good.

b. Find an infinite good set disjoint from $S$.

(Two sets are disjoint if they have no common elements.)