I am trying to prove that the vector-valued function $$\vec{R}(t)=e^{-t}\hat{i}+\ln t\hat{j}+\frac{1}{\sqrt{4-t}}\hat{k}$$ is continuous on its domain. I know that its domain is $\mathbb{R}\cup \mathbb{R}^{+}\cup(-\infty,4)$, or basically $(0,4)$.
I am getting rusty in terms of proving that a function is continuous, so please bear with me. As for continuity, as I have learned from our past lessons, there are three conditions.
- $\lim_{t\to a}f(t)$ exists for all $t$ on its domain.
- $f(a)$ exists for all $t$ on its domain.
- $\lim_{t\to a}f(t)=f(a)$.
However, there are parts on which this definition doesn't hold. Let $g(t)=\ln t$. We have $$lim_{t\to 0^{+}}g(t)=lim_{t\to 0^{+}}\ln t=-\infty$$.While this holds for limits, I don't think it will work for $g(0)$.
Despite that, I think it's "reasonable" to say that $g(0)=-\infty$ since the natural logarithm function is not defined for all $t<0$, and it decreases without bound. But we know that it's a mathematical sin to equate infinity to any function.
The same problem is encountered on $\frac{1}{\sqrt{4-t}}$, where $t=4$.
I think I'm doing something wrong here. Perhaps, I'm doing it all wrong. I think there are theorems that explain the continuity of exponential, logarithmic, and rational functions. But I also think our professor wouldn't give a trivial question where we will only need to find the union of the domains of the functions representing the whole of the vector-valued function.