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 limxexlogx=0 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 ddzlog(z)=1z, and so

limxlog(x)ex


=limxlog(x)ex

Where we let xx. Now we apply L'Hôpital to get
=limx1xex

Clearly the denominator grows without bounds, and thus the fraction approaches zero.



Another approach is to let z=reiϕ and note that log(z)=log(r)+i(ϕ+2kπ). Here we can write this as log(x)=log(x)+(2k+1)πi because we have that x is a positive real. By noticing the growth of each each term is less than ex 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 R>0 to R (which we can further generalize to let x be an element of C)


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