Prime Numbers

ƒ(x)

Ok, here's a challenge for you guys.

Lets figure out a pattern for prime numbers.

HallsofIvy

Right. Get back to you with the solution as soon as I have a moment.

Hurkyl

Ooh, I've figured it out. The prime numbers appear precisely at those integers that have exactly two positive factors!

ƒ(x)

Do you mean besides 1 and the number itself?

ƒ(x)

Um, correct me if I'm wrong, but prime numbers don't have factors.

epkid08

How can a number not have factors?

CRGreathouse

Chime in, everyone:

A prime number is divisible by precisely two positive factors, one and itself.

CRGreathouse

They're all either of the form 30a + b, where b is in {1, 7, 11, 13, 17, 19, 23, 29}, or in the 'exceptional set' {2, 3, 5}.

dodo

Of course, a lot of non-primes like 49 or 77 fit the pattern as well. :)

There should be some sort of FAQ on these forums, since this subject (and others) repeat very often, and I believe you (CR) posted a 'prime formula' just a few months ago.

Nor I can say I understand the OP's motivation. This ain't no circus, yo.

Almanzo

Prime numbers have exactly two (positive integer) divisors: 1, and the number itself. after all, N = 1 * N, and N = N * 1.

As far as I know, some patterns have been found which generate only prime numbers, but no pattern has been found which generates all of them. In general, to see if some large numer is prime, one has to try all possible divisors. (In practice some divisors, such as 2, 3 and 5, are readily discernible if the number is written in base 10.)

One interesting pattern is the following. If the last prime number found is M, calculate N = M! + 1, or 1 * 2 * 3 *....*(M-1) * M + 1. Now, either N is itself prime, or else it has a prime divisor larger than M. This recipe generates an infinite number of primes, therefore proving that there is no largest prime.

Starting with 1, the recipe gives the sequence 2, 3, 7, 71... and already misses 5.

CRGreathouse

Asymptotically, almost all numbers of the form I posted are composite.

The only people who would read the FAQ are those who don't need to read it.

I did post a formula for primes not too long ago.

It's widely-believed, though absolutely false, that it is a 'great unsolved problem' in math to find patterns in prime numbers or a formula for primes. Amusingly, little is further from the truth -- any person can easily find patterns in the primes, and formulas/algorithms for the primes are a dime a dozen.

ramsey2879

Show us an infinite pattern which generates ONLY PRIME numbers and you will be famous.

Hurkyl

When do I get my accolades?

CRGreathouse

My mousepad (an old sheet of scrap paper) lists nine different infinite patterns that generate only prime numbers: the sieves of Eratosthenes, Pritchard (x2), Dunten-Jones-Sorenson, Atkin-Bernstein, Galway (x2), and Sorenson, along with Bernstein's version of AKS. (It also mentions the Miller-Rabin test, but that generates infinitely many composites -- though it still produces mostly primes.) I could probably list ten more prime-generating patterns/methods/algorithms off the top of my head.

Admittedly, all of these people are famous to some degree. But less complicated or worthwhile methods are created all the time. Further, there are methods (like the LL test for Mersenne primes) that are conjectured to produce infinitely many primes but no one has proven it yet.

dodo

I believe the 'great unsolved problem' is a closed form for the sequence of primes. This thread won't rest in peace until you copy it *again*, or paste a link, or something.

In the meantime, Google is wise, Google is good. http://mathworld.wolfram.com/PrimeFormulas.html

CRGreathouse

Yes, because an amazing achievement like Sorenson's pseudosquare sieve (deterministically detecting primes in log-squared space as fast as Miller-Rabin!) is somehow less meaningful than some computationally worthless double summation based on Wilson's theorem.

nomentanus

There are a number of algorithms to derive primes, it goes without saying - but these don't resolve to, or immediately suggest any simple predictable "pattern". Yet the distribution of the primes does have what seems to be apparent "pattern", it could be said, anyway. Against this, Mandlebrot patterns aren't apparent by just glancing at the equation for that set, either.