How many positive integers are there that are divisible by and that have exactly divisors (1 and the integer itself included)?
Solution
Let be a positive integer that is divisible by and that has exactly positive divisors. Since , also should be divisible by these four primes. Thus, , where and is not divisible by any of the primes . All the factors of can be expressed as , where , and is a factor of . There are choices for (from to ) and similarly, there are and choices for and , respectively. Therefore, has different factors, where stands for the number of factors of . We require . As , and , we see that each of these numbers is divisible by some prime numbers and the number can thus be expressed as a product of at least four prime numbers. But as itself is a product of exactly four prime numbers, we conclude that , and are exactly those primes , and , in some order, and . From the latter condition we see that because any numbers bigger than has more than one factor. So for to satisfy the conditions, must be expressible as , where are the numbers , and in some order. Thus there are numbers satisfying the conditions.