lower bound for the prime number function

Question

Does there exist a function $f$ that is a lower bound of the prime number function $\pi$ with $f \sim \pi$?

Just to be a little puckish, I will note that $$f(x)=\pi(x)-1$$ satisfies your requirements :) — Zev Chonoles, Aug 23 '11 at 18:10
$pi(x)$ is the prime number counting function, not the prime number function — Smiley1000, Apr 23 '20 at 15:29

André Nicolas · Answer 1 · 2011-08-24T11:08:50.630

10

This segment of the Wikipedia article mentions a few such bounds. In all cases, the bounds hold from a certain explicit $N$ on. They can be easily be made unconditional without changing the asymptotic behaviour by suitably redefining the functions for $x \lt N$.

For instance, the article mentions the bounds $$\frac{x}{\ln x+2}<\pi(x)<\frac{x}{\ln x-4},$$ valid for $x \ge 55$, as well as much stronger bounds by Pierre Dusart. Since $\pi(x)$ is asymptotically $x/\ln x$, the two bounds above have the right asymptotic properties. Tweaking the lower bound so that it is valid below $55$ is easy. The crudest method is to use the function which is $-1$ for $x \lt 55$, and $x/(\ln x +2)$ for $x \ge 55$. Asymptotic behaviour is unaffected.

edited Aug 24 '11 at 11:08

answered Aug 23 '11 at 17:44

André Nicolas

507,029

1

See also http://math.stackexchange.com/questions/54312/non-trivial-upper-bound-for-the-number-of-primes-less-or-equal-to-n/54325#54325. – lhf Aug 23 '11 at 18:20
1

I wanted to add the article of Dussart. – davidlowryduda Aug 24 '11 at 11:05
@mixedmath: Thanks for the helpful addition. – André Nicolas Aug 24 '11 at 11:10

score 3 · Accepted Answer · edited May 04 '22 at 20:14

3

I found a more 'elegant' lower bound:

$$ \frac{n}{\log\,n} - 2 \leq \pi(n), \; n \geq 2 $$ Primality Testing in Polynomial Time

edited May 04 '22 at 20:14

The Amplitwist

795

answered Sep 24 '11 at 01:19

There is a proof on page 46: http://www.springerlink.com/content/p9rft7x5nkbwjkq6/ – Sep 24 '11 at 01:41
Sorry, was thinking of $\text{li}(x)$, that bounces back and forth from below $\pi(x)$ to above. – André Nicolas Sep 24 '11 at 01:49
Although, I am not sure what is meant by $\log$ in the paper. The author distinguishes between $\ln$ (natural logarithm) and $\log$ (base 2 or base 10, maybe?)... – Sep 24 '11 at 01:57
On second or third thought, I am again very surprised at the result, and doubtful. Maybe there is confusion with estimates of $p_n$. I do not trust Wolfram Mathworld assertions, they are often wrong. – André Nicolas Sep 24 '11 at 06:18
mmh, I don't think so, Martin Dietzfelbinger seems like a reliable source. – Sep 24 '11 at 12:20

lower bound for the prime number function

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