Saturday, 8 March 2014

real analysis - What is $ lim_{x to 0} log_0(x) $?



As per the title; what is $ \lim_{x \to 0} \log_0(x) $ ?



According to WolframAlpha: $$ \lim_{x \to 0} \log_0(x) = 0 $$ but how is this possible?



Surely the limit should be indeterminate since $\log_0(x) = \frac{\log(x)}{\log(0)} $ and $ \log(0) = $ indeterminate?


Answer



$\lim\limits_{x\to0}\log_0 x$ cannot exist unless $\log_0 x$ exists for $x$ in some open neighborhood of $0$, with the possible exception of $x=0$. (Since the base must be positive, we can take "open neighborhood of $0$" to mean a set of the form $[0,\varepsilon)$ where $\varepsilon>0$.)




Later edit: I suspect what's going on is something like this:
$$
\log_\varepsilon x = \frac{\log x}{\log\varepsilon},
$$
then letting $\varepsilon\downarrow0$, we have the denominator going to $-\infty$, so that the fraction approaches $0$. Hence $\log_0 x$ get construed to be $0$, and then one lets $x$ go to $0$, and the limit is $0$.


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