Are You A Prime Number

Question

I\'ve been interested in the problem of finding a better prime number recognizer for years. I realize this is a huge area of academic research and study - my interest in this i

Answer 1

Your code there only has complexity O(sqrt(n)lg(n)). If you assume basic mathematical operations are O(1) (true until you start using bignums), then it's just O(sqrt(n)).

Note that primality testing can be performed in faster-than-O(sqrt(n)lg(n)) time. This site has a number of implementations of the AKS primality test, which has been proven to operate in O((log n)^12) time.

There are also some very, very fast probalistic tests - while fast, they sometimes give an incorrect result. For example, the Fermat primality test:

Given a number p we want to test for primality, pick a random number a, and test whether a^(p-1) mod p = 1. If false, p is definitely not prime. If true, p is probably prime. By repeating the test with different random values of a, the probability of a false positive can be reduced.

Note that this specific test has some flaws to it - see the Wikipedia page for details, and other probabilistic primality tests you can use.

If you want to stick with the current approach, there are a number of minor improvements which can still be made - as others have pointed out, after 2, all further primes are odd, so you can skip two potential factors at a time in the loop. You can also break out immediately when you find a factor. However, this doesn't change the asymptotic worst-case behavior of your algorithm, which remains at O(sqrt(n)lg(n)) - it just changes the best case (to O(lg(n))), and reduces the constant factor by roughly one-half.