In number theory, Goldbach's weak conjecture, also known as the odd Goldbach conjecture or the 3-primes problem, states that:
- Every odd number greater than 5 can be expressed as the sum of three primes.
This conjecture is called "weak" because Goldbach's strong conjecture concerning sums of two primes, if proven, would establish Goldbach's weak conjecture. (Since if every even number >4 is the sum of two odd primes, merely adding three to each even number >4 will produce the odd numbers >7.)
The conjecture has not yet been proved, but there have been some helpful near misses. In 1923, Hardy and Littlewood showed that, assuming a certain generalization of the Riemann hypothesis, the odd Goldbach conjecture is true for all sufficiently large odd numbers. In 1937, a Russian mathematician, Ivan Vinogradov, was able to eliminate the dependency on the Riemann hypothesis and proved directly that all sufficiently large odd numbers can be expressed as the sum of three primes. Although Vinogradov was unable to say what "sufficiently large" actually meant, his own student K. Borodzin proved that 314,348,907 is an upper bound for the threshold that determines if a number is large. This number has more than six million digits, so checking every number under this figure would be impossible. Fortunately, in 1989 Wang and Chen lowered this upper bound to 1043,000. If every single odd number less than 1043,000 is shown to be the sum of three odd primes, the weak Goldbach conjecture is effectively proved! However, the exponent still needs to be reduced a good deal before it is possible to simply check every single number.
In 1997, Deshouillers, Effinger, Te Riele and Zinoviev showed that the generalized Riemann hypothesis implies Goldbach's weak conjecture. This result combines a general statement valid for numbers greater than 1020 with an extensive computer search of the small cases.
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