TITLE: What would be the immediate implications of a formula for prime numbers?
QUESTION [9 upvotes]: What would be the immediate implications for Math (or sciences as a general) if someone developed a formula capable of generating every prime number progressively and perfectly, also able to prove (or disprove) the primality of every N-th number. I know this is a very large and subjective answer, however, I would like to know some of these implications - like the breaking of many security systems based on the primes. Moreover, there are examples of practical Math's implication, not just theoretical, of a possible prime number formula discovery? There is for example in Physics, Chemistry, Geography or Astronomy any field which would be very improved with a so great and dreamed Eureka?

REPLY [8 votes]: As others have mentioned there are many formulas for primes. 
I can't pass up the opportunity to mention my favorite:
$$p_n=1+\sum^{2^n}_{m=1}\left\lfloor \sqrt[n]n \left( \sum^{m}_{x=1}\left\lfloor \cos^2\left( \pi \frac{(x-1)!+1}{x}\right) \right\rfloor \right)^{-1/n} \right\rfloor$$
Maybe it's just my favorite because it's so complicated and unwieldy! 
I first found the formula in Hacker's Delight by Warren. The formula is cited as "Willan's Formula". The formula can be derived from the fact that $(x-1)!\equiv -1 (\mod x)$ iff x is 1 or prime. So if $((x-1)!+1)/x$ is an integer, $x$ is prime (or 1), so the cosine of this times $\pi$ will be -1 or 1 iff $x$ is prime or 1. $\cos^2$ gets rid of the negative, floor keeps only the values for which the $\cos^2$ is 1. 
This formula appears to be described on the mathworld page for prime formulas.