# A ratio connected to the distribution of primes

According to the Prime Number Theorem, a number $$n$$, roughly speaking, has probability of primality $$\sigma_n:=1/\ln n$$.

As every schoolchild learns, one can test the primality of $$n$$ by looking for divisibility by primes up to $$\sqrt{n}$$. If these divisibility tests enjoyed independence, one would get probability of primality" equal to $$\tau_n:=\prod_{p\leq \sqrt{n}} \frac{p-1}{p}\ .$$

Probability interpretations aside, one can examine the ratio $$\sigma_n/\tau_n$$. What limit, if any, does it approach? And how fast?

• See the Mertens theorems as usual the optimal asymptotic depends on the Riemann hypothesis. If your question is if we find $\pi(x) \approx \frac{x}{\ln x}$ from $\prod_{n \le \sqrt{x}} (1-n^{-1})^{\pi(n) - \pi(n-1)} \approx \frac{\pi(x)}{x}$ the answer is yes. Making $\approx$ precise should give the Mertens theorem. – reuns May 16 at 22:20
• theorem 429 in Hardy and Wright, the product for primes $p \leq t$ of your $(p-1)/p$ is about $e^{- \gamma}/ \log t$ This is due o Mertens – Will Jagy May 16 at 22:21