Every "large" even number may be written as 
where 
is a prime and 
is the set of primes
and semiprimes 
.
Chen's Theorem
See also
Almost Prime, Chen Prime, Goldbach Conjecture, Prime Number, Schnirelmann's Theorem, SemiprimeExplore with Wolfram|Alpha
References
Chen, J. R. "On the Representation of a Large Even Integer as the Sum of a Prime and the Product of at Most Two Primes." Kexue Tongbao 17, 385-386, 1966.Chen, J. R. "On the Representation of a Large Even Integer as the Sum of a Prime and the Product of at Most Two Primes. I." Sci. Sinica 16, 157-176, 1973.Chen, J. R. "On the Representation of a Large Even Integer as the Sum of a Prime and the Product of at Most Two Primes. II." Sci. Sinica 16, 421-430, 1978.Hardy, G. H. and Wright, W. M. "Unsolved Problems Concerning Primes." Appendix §3 in An Introduction to the Theory of Numbers, 5th ed. Oxford, England: Oxford University Press, pp. 415-416, 1979.Ribenboim, P. The New Book of Prime Number Records. New York: Springer-Verlag, p. 297, 1996.Rivera, C. "Problems & Puzzles: Conjecture 002.-Chen's Conjecture." http://www.primepuzzles.net/conjectures/conj_002.htm.Ross, P. M. "On Chen's Theorem that Each Large Even Number has the Form
or 
." J. London Math. Soc. 10, 500-506,
1975.Referenced on Wolfram|Alpha
Chen's TheoremCite this as:
Weisstein, Eric W. "Chen's Theorem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/ChensTheorem.html