We call a natural number a *twin* if it has two natural divisors whose difference is equal to . Determine whether there are more twin numbers or the numbers that are not twin among the first natural numbers.
Solution
Let be the number of twin numbers that do not exceed . Then , since all the numbers from and are twin, as they have divisors and respectively, and consists of all the numbers that belong to both and . But there are also twin numbers that do not exceed and do not belong neither to nor to , for example , and so we even have that . Since is divisible by , we obtain:
which precisely means that there are more twin numbers than the numbers that are not twin among the first natural numbers.
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