In the famous Chen's Theorem which states that every sufficiently large even positive integer $n$ can be written as $n = p + q$, where $p$ is a prime and $q$ is a product of at most two primes. This is the precise 'almost prime' definition we use, where it's either a prime or a product of two primes. Now from the prime number theorem we have a crude estimate for the size of the $n$th prime, namely $p_n \sim n \log(n)$. Are there any similar estimates for the set of almost primes? To be more precise, let $\mathcal{P}$ denote the set of primes, and let $\mathcal{P}^2$ denote the set {$pq : p,q$ primes}. Set $Q = \mathcal{P} \cup \mathcal{P}^2$ and enumerate the elements of $Q$as $a_1, a_2, \cdots$ The question is do we have an approximation for $a_n$?
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The number of almostprimes up to $x$ is asymptotic to $x\log\log x/\log x$. It doesn't matter whether you include the primes or not, as there are only $x/\log x$ of them. I think this gives $n\log n/\log\log n$ as a crude estimate for the $n$th almost prime. 

