\item Obtain the one-to-one function $f_1$ and $f_2$ by cutting the graph of $f$ at 
      a certain point ($x_1$, $y_1$) so that domain of ($f_1$)=($-∞$ , $x_1$] and 
      domain ($f_2$)=[$x_1$,$+∞$)
  • 59
    You need \infty
    – percusse
    Sep 18, 2013 at 15:00
  • 2
    When using XeLaTeX or LuaLaTeX as your typesetter, you need to include the »unicode-math« package. Sep 18, 2013 at 15:07
  • 1
    Just a general tip: It's safter to use \(f_1\) and \[f_1\] than $f_1$ and $$f_1$$. For more information, see this question: tex.stackexchange.com/questions/503/why-is-preferable-to or read about it in l2tabu.
    – Sam Whited
    Sep 18, 2013 at 15:11
  • 1
    @SamWhited -- I would disagree with you on the claim that there's a generic safety related advantage to using \( ... \) rather than $ ... $ to delimit inline math. Indeed, since \(and \) are not "robust" commands (in the LaTeX sense of the word "robust"), it's perilous to use them in the arguments of "moving" commands; no such difficulties arise with $. Note that the link you provide regards the use of $$ -- a rather different matter.
    – Mico
    Sep 18, 2013 at 15:58
  • @Mico Fair point; l2tabu doesn't mention \(\) either, though I had thought it talked about line spacing a bit. Oh well, ignore my comment (although I'd still go with the LaTeX way unless you need to put it in a moving argument for some reason).
    – Sam Whited
    Sep 18, 2013 at 16:20

2 Answers 2


A better style is:

Obtain the one-to-one function $f_1$ and $f_2$ by cutting the graph of $f$ at a certain point $(x_1, y_1)$ so that domain of $(f_1)=(-\infty , x_1]$ and domain $(f_2)=[x_1,+\infty)$

(Please compare the obtained spacing).


If you are still faced with such a problem, as the last resort, you can use the rotated eight as follows.

$(-\infinity, x_1]$

enter image description here

  • 1
    Only meticulous readers will spot the difference. Sep 18, 2013 at 15:38
  • 6
    Every good math font has the symbol corresponding to \infty.
    – egreg
    Sep 18, 2013 at 15:44
  • 3
    @egreg: Yes. It is just for emergency purposes. :-) Sep 18, 2013 at 15:47
  • Some fonts have a \infty that really looks like a rotated 8 instead of a distinct design.
    – lblb
    Oct 14, 2017 at 15:48

Not the answer you're looking for? Browse other questions tagged .