In the paper of Green and Tao "Restriction Theory of the Selberg Sieve, with applications," their theorem 6.1 states: Let $N$ be a large integer. Then the number of Chen primes in the interval $(N/2,N)$ is at least $c\_1N/\ln^2N$, for some absolute constant $c\_1>0$. 


My question is, what the heck is $c\_1$? Is it Brun's constant, or is that just wishful thinking?



Distribution of Chen primes.
Although I don't have the reference convenient, I believe that the last chapter of Halberstam and Richert's book Sieve Methods states (and proves) Chen's theorem with an explicit value of c\_1. 


As I recall, it is roughly 3/11 times the "expected" constant from probabilistic arguments.


