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I've solved for it making a computer program, but was wondering there was a mathematical equation that you could use to solve for the nth prime?

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4 Answers 4

up vote 29 down vote accepted

No, there is no known formula that gives the nth prime, except artificial ones you can write that are basically equivalent to "the $n$th prime". But if you only want an approximation, the $n$th prime is roughly around $n \ln n$ (or more precisely, near the number $m$ such that $m/\ln m = n$) by the prime number theorem. In fact, we have the following asymptotic bound on the $n$th prime $p_n$:

$n \ln n + n(\ln\ln n - 1) < p_n < n \ln n + n \ln \ln n$ for $n\ge{}6$

You can sieve within this range if you want the $n$th prime. [Edit: There are better ideas than a sieve, see the answer by Charles.]

Entirely unrelated: if you want to see formulae that generate a lot of primes (not the $n$th prime) up to some extent, like the famous $f(n)=n^2-n+41$, look at the Wikipedia article formula for primes, or Mathworld for Prime Formulas.

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It should be noted that this is derived from the prime number theorem. –  BlueRaja - Danny Pflughoeft Jul 30 '10 at 20:40
    
Right. I had omitted mentioning it, but thanks to your reminder I went and found a precise bound I hadn't previously seen. :-) –  ShreevatsaR Jul 30 '10 at 20:58

There are formulas on Wikipedia, though they are messy. No polynomial $p(x)$ can output the $n$th prime for all $n$, as is explained in the first section of the article.

There is, however, a polynomial in 26 variables whose nonnegative values are precisely the primes. (This is fairly useless as far as computation is concerned.) This comes from the fact that the property of being a prime is decidable, and the theorem of Matiyasevich.

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Note that this 26-variable polynomial isn't well suited to generate primes, as most of its codomain is negative. –  stevenvh Aug 7 '10 at 17:39
    
I remember it was presented to us in the elementary course of number theory, and I think the teacher said that after much work they managed to reduce it to 22 variables. –  Asaf Karagila Sep 11 '10 at 17:59
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This 26-variable polynomial still gives me the creeps. I find it very strange indeed. –  Sputnik Jun 7 '11 at 22:45

Far better than sieving in the large range ShreevatsaR suggested (which, for the 10^15-th prime, has 10^15 members and takes about 33 TB to store in compact form), take a good first guess like Riemann's R and use one of the advanced methods of computing pi(x) for that first guess. (If this is far off for some reason -- it shouldn't be -- estimate the distance to the proper point and calculate a new guess from there.) At this point you can sieve the small distance, perhaps just 10^8 or 10^9, to the desired number.

This is about 100,000 times faster for numbers around the size I indicated. Even for numbers as small as 10 to 12 digits this is faster if you don't have a precomputed table large enough to contain your answer.

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Thanks. I've updated my answer to not recommend sieving. –  ShreevatsaR Sep 9 '10 at 7:30
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@ShreevatsaR: Don't get me wrong -- your answer is far better than the naive method of stepping through numbers and testing them! I was just suggesting an improvement. :) –  Charles Sep 9 '10 at 18:55

No such formula is known, but there are a few that give impressive results. A famous one is Euler's: $$P(n) = n^2 − n + 41$$ Which yields a prime for every natural number lower than $41$, though not necessarily the $n$th prime.

See more here.

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