# How to determine the prime numbers? [closed]

What is the best way to determine the prime numbers? Is there a way other than trial-and-error to determine them? Is the set of prime numbers finite or infinite?

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Have you considered looking at the documentation? Something like typing `Prime and press F1? Prime – Öskå Dec 14 '13 at 19:52
I am asking this because all algorithms I found are based on trial-and-error... Is there an other way to determine them? – Claudia Dec 14 '13 at 19:53
@Claudia You are asking a very, very, very well-researched question that mathematicians have grappled with for centuries. Mathematica is not going to solve this question for you. Wikipedia is a good start. – user3680 Dec 14 '13 at 19:56
There are infinitely many prime numbers. Mathematicians have been able to prove this since at least c. 300 BC where a proof was written in Euclid's Elements. Some call this theorem Euclid's Lemma. Your first question is not quite clear to me in light of this fact. – J. W. Perry Dec 14 '13 at 20:55
Regarding your first question, there are many algorithms as you have probably seen in your research. In general they all involve some sort of "testing" procedure. As a result determining if a very large number is prime, or finding the next large prime is currently a computationally expensive task at best. Here is a partial list of primality tests. In particular see AKS. – J. W. Perry Dec 14 '13 at 21:15

I am not sure what are you asking for but there is a famous and very simple algorithm called the Sieve of Eratosthenes that gives you all the existing primes to a certain $n\in \Bbb N$.

There is also a theorem that says: "If $n$ is a composed integer, then it has a prime factor that does not exceed $\sqrt{n}$". So, for example, if we want to verify if $103$ is prime we would search for the primes that do not exceed $\sqrt{103}$ wich are $2,3,4,5,7$. Then we would verify if they are factors of $103$. Because they are not, then there is no prime factor that exceeds $\sqrt{103}$, thus $103$ is not a composed integer, meaning that is a prime number.

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My question was not precise due to my lack of knowledge on the algorithms, and "best" is relative. In my search I found so many algorithms to calculate the primes that I was kind of lost and all of them seemed to envolve some kind of testing. But now have all the answers I wanted: I know, according to Euclid's theorem, that there are infinitely many prime numbers (against my intuition). Among the methods I liked in special Sieve of Eratosthenes which is very simple and gives all the primes for a chosen n (of Naturals). – Claudia Dec 15 '13 at 0:49