Thursday, November 15, 2018

Indeterminate form of infinity over 0?



I know that indeterminate forms exist in limits, such as $\frac{0}{0}$, $\frac{\infty}{\infty}$, $0^0$, $\infty^0$, $1^\infty$.



Then, if $\lim\limits_{x \to a} p(x)=\infty$ and $\lim\limits_{x \to a} f(x)=0$, can we call $\lim\limits_{x \to a} \frac{p(x)}{f(x)}$ an indeterminate form of type $\frac{\infty}{0}$? Or does it not exist since it has $0$ for the denominator?


Answer




This is not an indeterminate form, because it's clear what happens. If $f(x)$ approaches $0$ from above, then the limit of $\frac{p(x)}{f(x)}$ is infinity. If $f(x)$ approaches $0$ from below, then the limit of $\frac{p(x)}{f(x)}$ is negative infinity. If $f(x)$ keeps switching signs as it approaches zero, then the limit of the quotient fails to exist.



There's no "tug-of-war" here, like you have with indeterminate forms.


No comments:

Post a Comment

analysis - Injection, making bijection

I have injection $f \colon A \rightarrow B$ and I want to get bijection. Can I just resting codomain to $f(A)$? I know that every function i...