Find all natural numbers for which the number of positive divisors of is a power of 2.
Solution
For each prime , the numbers of the interval are not solutions, because the degree of in the decomposition of the LCM is 2 and so contributes by a factor of 3 to the number of divisors. Therefore this number is not a power of 2. From Bertrand's postulate for a prime there is a prime with , respectively for ; also . Therefore, each interval and for consecutive primes intersects. We obtained that and a direct check shows that only and are solutions.
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