4
$\begingroup$

Let us define $A_1$ to be a set of all numbers of the form $p_1+q(p_1)$ where $p_1$ goes through a set of primes $\mathbb P$ and $q(p_1)$ is $1$ if $p_1$ is even prime and $2$ if $p_1$ is odd prime.

Let us define $A_2$ to be a set of all numbers of the form $p_1p_2+q(p_1,p_2)$ where $(p_1,p_2)$ goes through a set $\mathbb P \times \mathbb P= \mathbb P^2$ and $ q(p_1,p_2)$ is $1$ if at least one of numbers $p_1$ and $p_2$ is even and $2$ if both are odd.

$$...$$

More generally, let us define $A_k$ to be a set of all numbers of the form $p_1...p_k+q(p_1,...,p_k)$ where $p_1...p_k$ goes through a set $\mathbb P^k$ and $q(p_1,...,p_k)$ is $1$ if at least one of numbers $p_1,...p_k$ is even and $2$ if all are odd.

It is more than expected that for every $k \in \mathbb N$ a set $A_k$ contains an infinite number of primes , but how to prove that? Is anything known already? Are there some results in this direction?

If there is an infinite number of twin primes then $A_1$ has an infinite number of primes, so a case of $A_1$ is probably open (because converse also holds), but is anything known about other cases or some particular case? Can it be shown by some analysis that at least one of these sets must contain an infinite number of primes, maybe by some inclusion-exclusion principles?

If we denote by $\text{small}(A_k)$ the smallest member of the set $A_k$ then $(\text{small}(A_k))_{k=1,2,...}$ is just our familiar sequence of primorials.

$\endgroup$

1 Answer 1

5
$\begingroup$

An easy modification of Chen's theorem gives that $p-2$ (for primes $p\ge5$ say) is infinitely often either prime or the product of two primes. This shows that either $A_1$ or $A_2$ contains infinitely many primes.

My understanding of the general parity phenomenon is that one could prove the statement "either $A_j$ or $A_k$ contains infinitely many primes" whenever $j$ is odd and $k$ is even (in theory—see Terry Tao's comment below). But in particular, I don't believe we can yet show that any particular $A_k$ contains infinitely many primes on its own.

$\endgroup$
2
  • 4
    $\begingroup$ To my knowledge, the claim "either $A_j$ or $A_k$ contains infinitely many primes" for arbitrary $j,k$ of different parity is currently only proven under the hypothesis of the Elliott-Halberstam conjecture (a result of Bombieri, mathscinet.ams.org/mathscinet-getitem?mr=396435). The parity phenomenon doesn't stand in the way of an unconditional proof, but this doesn't mean we can actually achieve such a proof, as there are other obstructions beyond parity. $\endgroup$
    – Terry Tao
    Mar 23, 2018 at 3:24
  • $\begingroup$ If $j=k+1$, the conjecture is also true. $\endgroup$
    – user178594
    Mar 17 at 13:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.