1

$$\lim\limits_{x\to\infty} \ \frac{\ln \ x}{x} = 0$$

How can I know this without looking at the graph? What's the easiest way to equate that expression to 0?

I understand that an integer over x approaches 0 as x approaches 0, but since ln x is an increasing function as x approaches infinity, how do we know which effect outweighs the other?

M.Mass
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StopReadingThisUsername
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  • Do you know l'Hospital? –  Jul 31 '17 at 07:56
  • Thanks for pointing me in some direction. I'll look into it. – StopReadingThisUsername Jul 31 '17 at 07:57
  • $\ln x=2\ln\sqrt{x}\le2(\sqrt{x}-1)$ for $x>0.$ –  Jul 31 '17 at 07:57
  • How do we know that $ln \sqrt{x} <= \sqrt{x} - 1$? – StopReadingThisUsername Jul 31 '17 at 07:59
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    Another way to think of the result is zero is that the increment of $\ln$ is very slow. Example, $x=1000$ $\ln x=6.907755279$ So, the denominator increases faster than the numerator. By it, we can conclude it tends to zeroes as $x$ tends to $\infty$ $\frac{\ln x}{x}=\frac{6.907755279}{1000}$. Logarithmic is a very slow increasing function. Its inverse is the exponential function which has a quite fast increasing property. – Crazy Jul 31 '17 at 08:13
  • It is a basic property of logarithms that $\log x \leq x-1$ for all $x>0$. If you know that $\log ab=\log a +\log b$ then you are also supposed to know the inequality properties of logarithm. – Paramanand Singh Jul 31 '17 at 12:10

5 Answers5

7

By L'Hospital rule

$$\lim_{x \rightarrow \infty}\frac{\ln x}{x}=\lim_{x \rightarrow \infty}\frac{\frac{1}{x}}{1}$$

$$\lim_{x \rightarrow \infty}\frac{\frac{1}{x}}{1}=\lim_{x \rightarrow \infty}\frac{1}{x}$$

$$\lim_{x \rightarrow \infty}\frac{1}{x}=0$$

Crazy
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7

Since $\ln t \le t$, for $t = \sqrt{x}$ we would have

$$\ln \sqrt{x}=\dfrac{1}{2}\ln x \leq \sqrt{x} \iff \ln x \leq 2\sqrt{x}$$

and, for $x>1$,

$$0\le\ln x\leq 2\sqrt{x}\iff 0\le\dfrac{\ln x}{x}\leq \dfrac{2}{\sqrt{x}}$$

With the squeeze theorem we have $\dfrac{\ln x}{x} \underset{x\to +\infty}{\longrightarrow}0$

Marcelo Ventura
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Stu
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5

$$\lim_{x\to \infty} {\ln x \over x} =\lim_{x\to \infty} \ln x^{1/x} = \ln\left( \lim_{x\to \infty} x^{1/x}\right) = \ln 1 = 0$$

Limit used : How to show that $\lim_{n \to +\infty} n^{\frac{1}{n}} = 1$?

user8277998
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4

With $x=e^t$ we have to investigate $\lim_{t\to \infty} \frac{t}{e^t} $. For $t>0$ we have

$0<\frac{t}{e^t}=\frac{t}{1+t+\frac{t^2}{2}+...} \le \frac{t}{\frac {t^2}{2}}=\frac{2}{t}$.

Your turn !

Fred
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4

A high school proof, using only basic properties of the integral:

For $t\ge 1$, we have $\sqrt t\le t$, so $\dfrac1t\le\dfrac 1{\sqrt t}$ and hence, if $x\ge 1$, $$ \ln x=\int_1^x\frac{\mathrm d\mkern1mut}{t}\le\int_1^x\frac{\mathrm d\mkern1mut}{\sqrt t}=2(\sqrt x-1)<2\sqrt x$$ from which we deduce at once $$0\le\frac{\ln x}{x}<\frac{2\sqrt x}{x},\quad\text{which tends to }0.$$

Bernard
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