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I have a concrete question and a more abstract version of the same. There are lots of theorems that take information on the number of elements and translate it to information on the primes; I’m looking to go the other way.

Abstract: I have an arithmetic semigroup generated by a collection of “primes” which have a known distribution function. What information do I need (e.g., about the small primes) to find the number of elements with norm at most $x$, and what is this?

Concrete: The primes are the odd primes $p$ of the natural numbers, and their valuations are $|p| = p/2$. Hence there are $\sim 2x/\log x$ primes up to $x$. How many elements are there up to $x$?

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    $\begingroup$ Aren't you just counting odd numbers, then? $\endgroup$ Commented Jul 17 at 23:03
  • $\begingroup$ @QiaochuYuan No, because the valuations are different. 27 < 15. $\endgroup$
    – Charles
    Commented Jul 18 at 0:04
  • $\begingroup$ It is hard to figure what you mean there are lots of theorems for. Can you give us examples? $\endgroup$ Commented Jul 18 at 0:14
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    $\begingroup$ Also, "the primes are the odd primes..." It is entirely unclear what you mean by that. What do you mean by the word "valuation?" $\endgroup$ Commented Jul 18 at 0:17
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    $\begingroup$ @Charles If you're going to make this distinction between a prime/element and its valuation, then you'll need to be more specific about your counting functions. Are you counting primes up to $x$, or primes whose valuations go up to $x$? elements up to $x$, or elements whose valuations go up to $x$? Is an element a product of primes or a product of valuations? If the former, is the valuation of a product of primes equal to the product of the valuations of the primes? $\endgroup$ Commented Jul 18 at 3:22

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To state the question more clearly, we are interested in $$ \#\{ n\in\Bbb N,\, n\text{ odd}\colon n/2^{\Omega(n)} \le x\} $$ where $\Omega(n)$ is the number of prime factors of $n$ counted with multiplicity. There is probably a standard method for attacking this type of problem directly (compare for example to classical results for $\#\{n\colon \phi(n)\le x\}$). But to orient ourselves, we can proceed by writing $$ \#\{ n\in\Bbb N,\, n\text{ odd}\colon n/2^{\Omega(n)} \le x\} = 1 + \sum_{w=1}^\infty \#\{ n\in\Bbb N,\, n\text{ odd}\colon \Omega(n) = w,\, n \le 2^wx \}. $$ Without the restriction to $n$ odd, this simply references the classical Selberg–Sathe result $$ \#\{ n\le y\colon \Omega(n)=w\} \sim G\biggl( \frac{w-1}{\log\log y} \biggr) \frac{y(\log\log y)^{w-1}}{(w-1)!\log y} $$ where $$ G(z) = \frac1{\Gamma(z+1)} \prod_p \biggl( 1-\frac zp\biggr)^{-1} \biggl( 1-\frac1p \biggr)^z. $$ We would then get a sum like $$ \sum_{w=1}^\infty G\biggl( \frac{w-1}{\log\log y} \biggr) \frac{2^wx(\log\log x)^{w-1}}{(w-1)!\log x} $$ (in practice $\log(2^wx)$ is close enough to $\log x$) which could be analyzed by standard means; indeed, the maximal summand occurs near $w\approx 2\log\log x$, in a region where the $G()$ value is basically constant, and we'd simply get the series for $\approx G(2) \frac x{\log x} e^{2\log\log x} \approx G(2) x\log x$.

I recommend going to a treatment of the Selberg–Sathe result (Montgomery & Vaughan Section 7.4, or Tenenbaum Chapter 2 Section 6.1) and modifying the method to restrict to odd numbers. One should be able to get an asymptotic formula of size $x\log x$, with the right leading constant even. [Technical note: the Selberg–Sathe result only holds as stated uniformly for $w\le(2-\delta)\log\log x$, which is just short of what we need. However, when restricted to odd integers, the range of uniformity will extend to $w\le(3-\delta)\log\log x$.]

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