Monday, 9 December 2013

real analysis - Is really :displaystylelimxtoinftyexlogx=0 and what it does meant this in wolfram alpha?



I have tried to know more about limit calculations about the product of the function f and it's inverse compositional f1 as example I have took the bellow example which mixed me in wolfram alpha , the function exlogx

defined over (0,+) and it has a limit equal's 0 at x= , as shown here in wolfram alpha , now my question here is





Question:
Is really lim and what it does meant this in wolfram alpha?




Answer



What does this mean? Well, if we let this be a complex valued function then we have that \frac{d}{dz} \log(z) = \frac 1z, and so

\lim_{x \to -\infty} \log(x)e^x
=\lim_{x \to \infty} \frac{\log(-x)}{e^x}
Where we let x \to -x. Now we apply L'Hôpital to get
=\lim_{x \to \infty} \frac{-1}{xe^x}
Clearly the denominator grows without bounds, and thus the fraction approaches zero.



Another approach is to let z = re^{i \phi} and note that \log(z) = \log(r) + i(\phi + 2k\pi). Here we can write this as \log(-x) = \log(x) +(2k+1)\pi i because we have that x is a positive real. By noticing the growth of each each term is less than e^x we can conclude with a little work.



To answer the OP more directly perhaps, what is happening here is the analytic continuation of the function \log(x) from \mathbb{R}_{>0} to \mathbb{R} (which we can further generalize to let x be an element of \mathbb{C} )


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