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What is the size of the largest $A\subseteq \{1,\ldots,N\}$ such that there is a function $\delta:A\to \{-1,1\}$ such that \[\sum_{n\in A}\frac{\delta_n}{n}=0\] and \[\sum_{n\in A'}\frac{\delta_n}{n}\neq 0\] for all $A'\subsetneq A$?
#319
:
[ErGr80]
number theory
,
unit fractions
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