Chen's theorem states that every sufficiently large even number can be written as the sum of either two primes, or a prime and a semiprime (the product of two primes).
On that page, it is also written: "Chen's theorem is a giant step towards the Goldbach conjecture and a remarkable result of the sieve methods."
I would like to know particularly about methods that Chen did use in his proof of the stated theorem.
Also, are there some books that focus mainly on sieve methods in number theory and can you recommend some?
Thanks.