When trying to solve a number theory problem I came across this other problem which sounds interesting. Let $n$ be a positive integer, and consider $n$ consecutive positive integers $a_1, \ldots, a_n$ that are at most $n^2$.

What is an upper bound for the number of integers in this kind of list that have a prime factor greater than $n$?

What's interesting is that for any such prime factor, it appears only once as a factor in the list, and there can only be at most $n$ such primes. I'm guessing that $n$ is too large an upper bound and cannot be reached, i.e. there is always at least one number with prime factors all less than or equal to $n$.

I don't have any results besides checking some lists of numbers, and I don't really know how to approach this. Any ideas?


At least one of the numbers will be divisible by $n$. It cannot have a prime factor greater than $n$ because then it would be at least $n(n+1)$, so at most $n-1$ of them can have a prime factor larger than $n$. An example that meets this is $n=5$ and the numbers $19-23$ where only $20$ does not have a prime factor greater than $5$.

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  • $\begingroup$ Silly me, I should have thought of that. Now I guess the next natural question is whether the bound $n-1$ can be achieved infinitely often. $\endgroup$ – Paul Cusson Jan 18 at 4:05

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