MathOverflow is a question and answer site for professional mathematicians. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Is there infinite number of prime pairs $(p_k,p_n)$ that satisfy equality $p_n=2p_k-3$

share|cite|improve this question
up vote 18 down vote accepted

This is surely open. The usual conjecture is that, given a collection of linear equations in primes, if there is no "local" obstacle to it having infinitely many solutions, then it does. Here local obstacles would mean either (1) the size of the real solutions is bounded or (2) there is some prime number $q$ such that any solution has at least one of the $p_i$ divisible by $q$. Green and Tao term this the "generalized Hardy-Littlewood conjecture".

Since neither of these obstacles applies, one would conjecture that there are infinitely many prime pairs of this form. Green and Tao have recently proved that this is true in many cases but your example is not among the cases to which their results apply; in the terminology of their paper, it has "complexity $\infty$". I am fairly certain no progress has been made on the complexity $\infty$ cases.

Note also that your conjecture is extremely close to the conjecture that there are infinitely many Sophie Germain primes, which remains notoriously open.

I highly recommend reading the introduction to Green-Tao for an overview of what can and cannot be done in this area.

share|cite|improve this answer
Surely you don't need to say "surely" in the first sentence. :) – KConrad Sep 5 '11 at 15:25
And don't call me "Surely" – Igor Rivin Sep 6 '11 at 9:48

maybe there is,it is not proved yet. It is a particular case of a general conjecture concerning linear equations in primes :

Conjecture : Let $a$ and $b$ two integers such that $a>0$, the product $ab$ is even and $a$ and $b$ are coprime. Then the equation $p' =ap+b$ has infinitely many solutions in pairs of prime $(p,p')$.

share|cite|improve this answer
This conjecture seems to be a special case of the general conjecture David Speyer mentioned; the general form of the conjecture goes by 'generalized Hardy--Littlewood conjecture.' – user9072 Sep 6 '11 at 9:47

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

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