ISL 2002 A6

Let $A$ be a non-empty set of positive integers. Suppose that there are positive integers $b_1,\ldots b_n$ and $c_1,\ldots,c_n$ such that

• for each $i$ the set $b_iA+c_i=\left\{b_ia+c_i\colon a\in A\right\}$ is a subset of $A$ , and

• the sets $b_iA+c_i$ and $b_jA+c_j$ are disjoint whenever $i\ne j$

Prove that $${1\over b_1}+\,\ldots\,+{1\over b_n}\leq1.$$

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