MHB Xcessive's question at Yahoo Answers regarding a limit

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The limit L of the expression x^ln(x) / (ln(x))^x as x approaches infinity is evaluated. The problem presents an indeterminate form of ∞/∞, allowing the application of L'Hôpital's Rule. Through implicit differentiation of both the numerator and denominator, it is shown that the denominator grows faster than the numerator. Ultimately, the limit is concluded to be zero, confirmed by an alternative approach using exponential functions. Thus, L equals zero as x approaches infinity.
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Here is the question:

A not so simple problem, or is it?

x^ln x/ (lnx)^x ; when x approaches infinity

I have posted a link there to this topic so the OP can see my work.
 
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Hello Xcessive,

We are given to compute:

$$L=\lim_{x\to\infty}\frac{x^{\ln(x)}}{\ln^x(x)}$$

Observing that we have the indeterminate form $$\frac{\infty}{\infty}$$, we may apply L'Hôpital's Rule. Thus, we need to compute the derivatives of the numerator and denominator.

Let's begin with the numerator, and let:

$$y=x^{\ln(x)}$$

Taking the natural log of both sides, we obtain:

$$\ln(y)=\ln^2(x)$$

Implicitly differentiating with respect to $x$, we have:

$$\frac{1}{y}\frac{dy}{dx}=\frac{2}{x}\ln(x)$$

$$\frac{dy}{dx}=\frac{2y}{x}\ln(x)$$

$$\frac{dy}{dx}=\frac{2x^{\ln(x)}}{x}\ln(x)=2x^{\ln(x)-1}\ln(x)$$

Now, for the denominator, let:

$$y=\ln^x(x)$$

Taking the natural log of both sides, we have:

$$\ln(y)=x\ln(\ln(x))$$

Implicitly differentiating with respect to $x$, we have:

$$\frac{1}{y}\frac{dy}{dx}=x\frac{1}{\ln(x)}\frac{1}{x}+\ln(\ln(x))=\ln(\ln(x))+\frac{1}{\ln(x)}$$

$$\frac{dy}{dx}=y\left(\ln(\ln(x))+\frac{1}{\ln(x)} \right)$$

$$\frac{dy}{dx}=\ln^x(x)\left(\ln(\ln(x))+\frac{1}{\ln(x)} \right)$$

And so we may now state:

$$L=\lim_{x\to\infty}\frac{2x^{\ln(x)-1}\ln(x)}{\ln^x(x) \left(\ln(\ln(x))+\frac{1}{\ln(x)} \right)}= 2L\lim_{x\to\infty}\frac{\ln(x)}{x\left(\ln(\ln(x))+\frac{1}{\ln(x)} \right)}$$

At this point, we may observe that the denominator dominates the numerator, and so we may conclude:

$$L=0$$
 
Another approach would be to write the limit as:

$$L=\lim_{x\to\infty}e^{\ln(x)(\ln(x)-x)}$$

Hence:

$$\ln(L)=\lim_{x\to\infty}\left(\ln(x)(\ln(x)-x) \right)=\lim_{x\to\infty}\left(\frac{\frac{\ln(x)}{x}-1}{\frac{1}{x\ln(x)}} \right)=-\infty$$

and so:

$$L=0$$
 
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