For $n,m \geq 3$, define $ P\_n = \{ p : p$ is a prime such that $ p\leq n$ and $ p \nmid n \}$ . 


For example :
$P\_3= \{ 2 \}$
$P\_4= \{ 3 \}$
$P\_5= \{ 2, 3 \}$,
$P\_6= \{ 5 \}$ and so on.


Claim: $P\_n \neq P\_m$ for $m\neq n$.


While working on prime numbers I formulated this problem and it has eluded me for a while so I decided to post it here. I am not sure if this is an open problem or solved one. I couldn't find anything that looks like it.
My attempts haven't come to fruition though I have been trying to prove it for a while. If $m$ and $n$ are different primes then it's clear. If $m \geq 2n$, I think we can find a prime in between so that case is also taken care of. My opinion is that it eventually boils down to proving this statement for integers that share the same prime factors. My coding is kind of rusty so would appreciate anybody checking if there is a counterexample to this claim. Any ideas if this might be true or false? Thanks.


PS: I asked this question on mathstackexchage and somebody recommended I post it here as well. Here is the link to the original post 


<https://math.stackexchange.com/questions/2536176/a-conjecture-regarding-prime-numbers>



A conjecture regarding prime numbers
This is true (for large $m$ and $n$) under ABC plus the assumption that there is a prime in $[x,x+x^{1/2-\delta}]$ for some positive $\delta$ (which is widely believed, but beyond RH). To see this, suppose $m <n$ 
and that they have the same radical $r$. Write $m=gM$ and $n=gN$ where $g$ is the gcd of $m$ and $n$, so that $M$ and $N$ are coprime. Applying the ABC conjecture to $M + (N-M) = N$, we conclude that 
$$ 
(N-M) r \ge N^{1-\epsilon},
$$ 
so that 
$$ 
n-m \ge n^{1-\epsilon}/r. 
$$ 
On the other hand, clearly $n-m$ is also $\ge r$ (since it is divisible by $r$). It follows that $n-m \ge n^{1/2-\epsilon}$, and the assumption that there are primes in short intervals finishes the (conditional) proof. 


The problem is likely very hard, as fedja's observation in the comments already shows. There is a conjecture of Hall that $|x^3-y^2| \gg x^{1/2-\epsilon}$ which is wide open. The best results that are known here (going back to Baker's method) are of the flavor $|x^3-y^2| \gg (\log x)^C$ for some $C$. If $x^3-y^2$ does get as small as in the Baker result, then take $n=x^4y$ and $m=xy^3$, which clearly have the same radical and then $|n-m|$ is of size essentially $n^{5/11}$. In other words, either you have to improve work towards Hall's conjecture, or work towards gaps between primes!


**Added** Thanks to Pasten's comment, I learned that this problem is already in the literature and is known as Dressler's conjecture. The conditional proof above is recorded in [work of Cochrane and Dressler](http://www.ams.org/journals/mcom/1999-68-225/S0025-5718-99-01024-8/S0025-5718-99-01024-8.pdf) who give more information on the conjecture.


