In number theory, Cramér's conjecture, formulated by the Swedish mathematician Harald Cramér in 1936, is an estimate for the size of gaps between consecutive prime numbers: intuitively, that gaps between consecutive primes are always small, and the conjecture quantifies asymptotically just how small they must be. It states that
where pn denotes the nth prime number, O is big O notation, and "log" is the natural logarithm. While this is the statement explicitly conjectured by Cramér, his argument actually supports the stronger statement
and this formulation is often called Cramér's conjecture in the literature. However, this stronger formulation is not supported by more accurate heuristic models, which nevertheless support the first version of Cramér's conjecture.
Neither form of Cramér's conjecture has yet been proven or disproven.
Conditional proven results on prime gaps
due to Baker, Harman, and Pintze.
In the other direction, E. Westzynthius proved in 1931 that prime gaps grow more than logarithmically. That is,
Cramér's conjecture is based on a probabilistic model (essentially a heuristic) of the primes, in which one assumes that the probability of a natural number of size x being prime is 1/log x. This is known as the Cramér model of the primes. Cramér proved that in this model, the above conjecture holds true with probability one.
Related conjectures and heuristics
In the Cramér random model,
However, as pointed out by Andrew Granville, Maier's theorem shows that the Cramér random model does not adequately describe the distribution of primes on short intervals, and a refinement of Cramér's model taking into account divisibility by small primes suggests that ( A125313), where is the Euler–Mascheroni constant.
In the paper  J.H. Cadwell has proposed the formula for the maximal gaps: which for large goes into the Cramer's conjecture .
He writes, “For the largest known maximal gaps, has remained near 1.13.” However, is still less than 1, and it does not provide support to Granville's refinement that c should be greater than 1.
- Prime number theorem
- Legendre's conjecture and Andrica's conjecture, much weaker but still unproven upper bounds on prime gaps
- Firoozbakht's conjecture
- Maier's theorem on the numbers of primes in short intervals for which the model predicts an incorrect answer
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