Wednesday, 21 February 2018

real analysis - Does limxrightarrow0fracln(x)cot(x) exist or not?



I stumbled on the following limit in a calculus textbook today:




limx0ln(x)cot(x)



According to the book's solutions and Mathematica, this limit exists and is equal to 0. I can see why 0 is obtained using l'Hôpital's rule twice:



...=limx0(1x)csc2(x)=limx0sin2(x)x=limx02sin(x)cos(x)1=2sin(0)cos(0)=0




If I recall correctly, l'Hôpital's rule is applicable when we have:
limxaf(x)g(x)


even if f and g are not derivable at precisely a, so there should be no issue in using it on the above limit.



However, I can't reconcile the fact that ln(x) is defined over ]0,+[ (and usually, only limx0+ln(x) exists) with the fact that the above limit exists (both as x0+ and as x0).



It seems to me that only




limx0+ln(x)cot(x)



should exist and thus the "bilateral limit" (with x0) does not exist since the limit with x0 doesn't.



Is there something I am missing?


Answer



Yes you are right, since lnx is defined for x>0 the limit for x0 is meaningless and only




limx0+lnxcotx



can be considered.



Note that this doesn't mean in general that the limit considered exists.



For example




limx0+lnxsin1x



can be considered but does not exist.


No comments:

Post a Comment

real analysis - How to find limhrightarrow0fracsin(ha)h

How to find limh0sin(ha)h without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...