Problem:
Let be a non-empty set of positive integers such that for any , all positive divisors of are also in . Prove that contains an integer of the form
where are distinct prime numbers, all greater than 2023.
Solutions — 2
Solution 1
Solution:
Since is non-empty, it contains some integer . Since , it follows that and thus . We call a pair of positive integers -valid if we have , and is divisible by distinct primes. We will show inductively that for every there is a -valid pair.
Lemma. If is -valid and is prime divisor of , then is -valid for all integers .
Proof. Using LTE, we have and hence . Also, since , we have . Because also , it follows that is -valid. Repeating times gives the desired result.
For , take . Now assume is -valid for some and let be a prime dividing . For any integer we have
Thus, the primes we know to divide make up at most a fraction
of . Using the lemma, we can pick arbitrarily large such that is still -valid. It follows that is divisible by a prime that does not already divide , hence is -valid. We conclude the solution by picking a -valid pair where is large enough to ensure is divisible by at least 2023 primes greater than 2023. We then use the lemma to make sure the powers of all these primes are in the prime factorisation of are at least 2023 .
Solution 2
Solution:
We start by defining the following sequence: and . For the same reason as in the first solution we have and therefore for all , so it's enough to show that an integer as in the problem statement will divide one of the . Now we also define . If is an integer it's odd (because is odd) and we can factorise:
and therefore:
is an integer as well. Since is an integer we therefore get by induction that all are integers and the equation above holds for all . Now assume we have a divisor of and look at the equation mod , we get , therefore:
If we apply this to some prime we realise that as soon as divides it divides not only but also for all , so is increasing and will eventually be bigger than 2023. Now let and ; since and because but we also get: but so . Apart of this is also true (again by induction) for all . So if there was only a finite amount of primes among the divisors of , we would get that is constant for all bigger than some , which would imply exponential growth, but grows much faster than exponential, so we can conclude that at some point a new prime will appear, which completes the proof.