9
$\begingroup$

When we talk about: $$\lim_{x\to{c}}f(x)=L$$ Is there a formal name for the number "$c$"?
I know of course that it means "$L$ is the limit of $f(x)$ as $x$ approaches $c$". It just would be nice to be able to refer to "$c$" separately without saying "what $x$ approaches". I tried to look at various textbooks but didn't find anything.

$\endgroup$
12
  • 6
    $\begingroup$ I am not aware of a standard terminology. $\endgroup$ Feb 5 '20 at 20:13
  • 2
    $\begingroup$ Just for fun: The "approachand," pronounced ah-proach-and, analogous to "operand." $\endgroup$ Feb 6 '20 at 0:36
  • 2
    $\begingroup$ I generally refer to it as "the limiting value of $x$". It is a little clunky, and probably not any better than "the thing which $x$ approaches", but works. $\endgroup$ Feb 6 '20 at 2:21
  • 2
    $\begingroup$ Maybe "approach value" for $c$ and "limit value" for $L$? $\endgroup$ Feb 6 '20 at 19:16
  • 5
    $\begingroup$ Maybe, we could refer to $c$ as the limit point because, in the definition of limit, $c$ has to be a limit point of the domain. This expression would be good because, according to the general definition, a limit point of the domain is a point that (i) can be approximated by elements of the domain and (ii) need not be in the domain (which are the two properties of $c$ that have to be highlighted for the students). $\endgroup$
    – Pedro
    Feb 26 '20 at 14:54
2
$\begingroup$

I vote for Pedro. We should call the point at which the limit is taken the limit point. In contrast, the value obtained by the limit (if it exists) is the limit's value. In particular, $$ \lim_{x \rightarrow c} f(x) = L $$ has limit point $c$ and the value of the limit is $L$. This terminology keeps with the usual usage of the term value for outputs of the function. In addition, while the term limit point does have a more abstract topological definition, I don't think there is much danger of confusion.

$\endgroup$
3
  • $\begingroup$ "The limit point of the limit"? $\endgroup$ May 29 '20 at 12:36
  • $\begingroup$ Yep. The limit point of the limiting operation. $\endgroup$ May 29 '20 at 15:33
  • $\begingroup$ If the co-domain is a topological space, then point could also refer to a point in the co-domain. I think this will normally not be an issue. But if we want to disambiguate, we might call c the limit point of the domain. $\endgroup$ Aug 6 at 7:53

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.