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Solve the inequality for $n\in\mathbb N$: $$\frac{n}{1,785\ln(\ln n)+\frac{3}{\ln(\ln n)}} < 8$$

Attempt:

After some algebra, $$\frac{n\ln(\ln n)-14,28\ln(\ln n)\ln(\ln n)-24}{1,785\ln(\ln n)\ln(\ln n)+3}<0$$

$$n\ln(\ln n)-14,28\ln(\ln n)\ln(\ln n)-24<0;1,785\ln(\ln n)\ln(\ln n)+3\neq 0$$

How to solve this inequality (without the use of computer)?

Note: I have found the inequality in this post

and I am using it to find all positive integers $n$ such that $\phi(n)=8$.

user300045
  • 3,449

1 Answers1

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This may not be a satisfactory answer for you.

The term $\log(\log(n))$ varies extremely slowly in the range $10\leq n\leq 50$. I computed its average value over that range and it is "almost" $1.2$. So, for a preliminary search, you could use $$\frac{n}{e^{\gamma } \log (\log (n))+\frac{3}{\log (\log (n))}}\approx \frac{n}{1.2e^{\gamma } +\frac{3}{1.2}}\approx 0.215n$$ which would give $n=37$ while the exact solution would be $n=37$ !!

In fact, you could perform a linear regression and obtain, for this range $$\frac{n}{e^{\gamma } \log (\log (n))+\frac{3}{\log (\log (n))}}\approx 0.220034 n-0.152421$$