mersenneforum.org > Math Asymptotic density of k-almost primes
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 2010-08-16, 20:12 #1 CRGreathouse     Aug 2006 5,987 Posts Asymptotic density of k-almost primes Of course this is $\pi_k(n)=\frac{n(\log\log n)^{k-1}}{(k-1)!\log n}+o\left(\frac{n(\log\log n)^{k-1}}{\log n}\right)$ for any fixed k, but are there better Li-type estimates, or at least more asymptotic terms (as Cipolla 1902 gives for $\pi(n)$)? For extra points: Are Dusart-style absolute error bounds known? Are there better asymptotic -- or better, Shoenfeld-style -- bounds known on the RH? This comes out of comparing the number of n-bit semiprimes to the number predicted by the formula with k = 2, where I noticed what seemed to be a consistent relative (reciprocal density) bias of about 0.5 over the range 15-50 bits.
 2010-08-22, 23:47 #2 wblipp     "William" May 2003 Near Grandkid 2×1,187 Posts You might try an Li version motivated by comparison to the prime case. When k=1 this is n/log(n), which we know to improved upon by integral (1/log(x)). This can be motivated by the argument that 1/log(x) is correct density. The analogous adjustment would be the integral of $\frac{(\log\log x)^{k-1}}{(k-1)!\log x}$

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