Is it possible that a set consisting of real numbers has exactly non-empty subsets such that the product of all elements of each subset is a rational number?
Solution
Answer: Yes.
Let be a positive integer whose value will be determined later on. Let be the set consisting of 's for . Let be the set of subsets of whose product of own elements is a rational number together with the empty set. Since every positive integer has a unique representation as sum of distinct powers of , the cardinality of is precisely the number of integers from to which are divisible by . Therefore,
Thus, if , then there exists an integer such that . As , there is such an . (Indeed, fulfills the condition.)
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