Wikipédia - https://en.wikipedia.org/wiki/Twin_prim ... conjecture
$\pi_2(x)$ označuje twin prime counting function, t.j. $\pi_2(x)$ je počet prvočísel $p$ takých, že aj $p+2$ je prvočíslo a $p+2\le x$.The Hardy–Littlewood conjecture is a generalization of the twin prime conjecture. Then the conjecture is that
$$
\pi_2(n) \sim 2 C_2 \frac{n}{(\ln n)^2} \sim 2 C_2 \int_2^n {dt \over (\ln t)^2}
$$
This conjecture can be justified (but not proven) by assuming that 1 / ln t describes the density function of the prime distribution, an assumption suggested by the prime number theorem.