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It seems to me that there are three ways to find large prime gaps without using brute force.

  • Natural Prime Order Using Factorials

For example, using $7!$, it is clear that $(7!+2), (7!+3), \dots (7!+8), (7!+9), (7!+10)$ are not primes.

Let $p_n$ be the $n$th prime. Around $p_n!+1$, there is a prime gap of at least length $p_{n+1}-1$.

This can be generalized to $a(b!)$ where $a \ge 1$ and $b \ge p_n$.

  • Natural Prime Order Using Primorials

The same pattern for factorials holds for primorials.

For example, using $7\#$, it is clear that $(7\#+2), (7\#+3), \dots (7\#+8), (7\#+9), (7\#+10)$ are not primes.

This can be generalized to $a(b\#)$ where $a\ge1$ and $b\ge p_n$.

  • Arbritrary Prime Order Using Chinese Remainder Theorem

For example, we can find a gap of at least $10$ using $4$ distinct primes say $2,3,7,11$.

This is equivalent to a CRT problem of finding $x$ where:

$x \equiv {-1} \pmod 2$

$x \equiv {-2} \pmod 3$

$x \equiv {-4} \pmod {11}$

$x \equiv {-6} \pmod 7$

Other numbers with this same prime gap can be found by adding factorials or primorials to $x$.

Here's my question:

Are these three methods the only way to find large prime gaps without using brute force? Are there any other methods that can be used?

Larry Freeman
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    Numberphile talked about and linked two papers about this quite recently, here and here, although I haven't read them thuroughly enough to see whether they're constructive. – Arthur Jul 28 '17 at 06:46
  • The real question is : are those gaps large ? The "mean gap" is $(n+1) \log(n+1) - n \log n \sim \log n$. There are not many results about the largest gaps – reuns Jul 28 '17 at 09:53
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    Looking at the form of the current top gaps might be illustrative: http://ntheory.org/gaps/top20.pl. This seems to be the most efficient current method for large gaps (it was used by many of the earlier searchers). Gapcoin uses brute force and is substantially less efficient than using the k * p# / d method. Below 2^64 the standard is exhaustive searches (Oliveira e Silva a few years ago and Mersenneforum's Prime Gap Search in 2017). – DanaJ Jul 31 '17 at 01:27

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