# How to quickly check if a number is prime? [closed]

Let say I've found a very very very long prime number. I know it's prime but I need to have a proof. Is there any fast way how to check if a number is really prime?

Let say I've found the longest prime number, longer than the longest known and I want to confirm it in very short time (seconds). Is it possible?

## closed as not a real question by Dominic Michaelis, J. M. is a poor mathematician, Joe, Alexander Gruber♦, PaulMay 1 '13 at 4:32

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• If you figure a way out let me know haha – Benzne_O Apr 22 '13 at 8:29
• How do you know it's a prime number if you don't have a proof? – Joel Reyes Noche Apr 22 '13 at 8:31
• If I knew an answer to this question, I'd probably be holding a Fields medal by now... – A.P. Apr 22 '13 at 8:41
• Ask the NSA or the CIA. If anybody knows, they do. – Neal Apr 22 '13 at 13:24
• The Lucas-Lehmer test on a CPU for a number of this magnitude takes about 6 days using optimized code. That is specific to Mersenne numbers, and for will be at least as fast as a Fermat test, which is the simplest decent compositeness (probable primality) test. So clearly our time scales are way off from what you would like, and we still just have a PSP, not a SPSP and certainly not a proven prime. Without some special form you're practically limited to ~40k digits (ECPP using Primo, a nice workstation/server, and a few months). This is rather small compared to the recent Mersenne primes. – DanaJ Aug 24 '17 at 19:21

If the number is of general form, fastest would be one of the modern versions of ECPP. This is several orders of magnitude faster than AKS.

If you can tolerate a tiny probability of error, the Miller-Rabin test works very well here.

If you can tolerate error, but only infinitesimally small, the Frobenius tests can provide much better worst-case error bounds than Miller-Rabin.

If the number is of special form the Lucas test or its variants might be applicable.

• For the purpose of this question -- a large number claimed to be prime, but without any evidence -- I highly recommend Miller-Rabin. A test might take a few months on a good computer if it's close to the maximum known, or longer if it's far beyond. But this is much better than the time it would take for a full proof. – Charles Apr 22 '13 at 18:09

I guess the Fastest method known till date is - AKS Primality Test

However it won't be able to find the longest prime number within seconds.

• This is an extremely slow test, even with Bernstein's modifications. – Charles Apr 22 '13 at 13:22