I ponder some numbers in bed, all products of three primes I've said, apply they're still fun: now Elev'n cubed plus one. What numbers could be in my head?
Solution
The numbers expressible as a product of three primes are each of the form , or , where , and are distinct primes. Now, , and . We require . The first case is easy to rule out, since necessarily or , which both fail. The second case requires , or . These give , and 2, respectively. As , we reject , but and . In the third case, exactly one of the primes is 2, since all other primes are odd. So say . There are three possibilities for , and . Those are , and , respectively, of which only is a pair of primes. So the third and final possibility is .
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