Goldbach's conjecture is one of the oldest unsolved problems in number theory and in all of mathematics. It states:
Every even integer greater than 2 can be written as the sum of two primes.
Expressing a given even number as a sum of two primes is called a Goldbach partition of the number. For example,
4 = 2 + 2
6 = 3 + 3
8 = 3 + 5
10 = 3 + 7 = 5 + 5
12 = 5 + 7
14 = 3 + 11 = 7 + 7
…
In other words, the Goldbach conjecture states that every even number greater than or equal to four is a Goldbach number, a number that can be expressed as the sum of two primes. See also Levy's conjecture.
Reference:
http://en.wikipedia.org/wiki/Goldbach%27s_conjecture
