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I'm helping my son with Project Euler and we're working on problem 7, "What is the 10001st prime number?" We'll use a Sieve of Eratosthenes and we'll increase the size of the initial array until we're left with 10001 primes. We'll start with a pretty big array and increase it by whatever seems reasonable, since there is no time constraint, until we get the answer.

My question is, is there a way to make an informed guess about the size of the initial array?

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up vote 10 down vote accepted

Wikipedia gives the bounds $n \ln n + n(\ln\ln n - 1) < p_n < n \ln n + n \ln \ln n$, where $p_n$ is the $n$th prime. So the $10001$st prime is between 104318 and 114320.

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Wow, I can't believe I haven't run into that before. –  uncle brad Jun 19 '11 at 13:07
@Luboš, when the question says that the OP is trying to solve a problem and just wants to allocate the right amount of memory to start with, it probably means he doesn't want to be told the solution. –  Peter Taylor Jun 19 '11 at 14:15
@Luboš Please delete your comment, it is a spoiler for those who still want to solve the problem. –  starblue Jun 19 '11 at 16:42

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