12

How could I type the symbol as shown in the picture below?

I tried to use $\mathbb{R}_{(-)}$ instead, but the - inside can not touch the parenthesis on both sides.

I want to create a horizontal line that stops automatically when it touches the parenthesis.

Enter image description here

2
  • 29
    Off topic: I never seen such symbol for usual topology on real line.
    – Sigur
    Apr 12 '20 at 14:33
  • More off-topic: perhaps that's a useful notation when used in conjunction with its counterpart having square brackets instead of round parentheses, so as to denote the "extended real line", that is, the two-point compactification of the real line. Otherwise, for the extended real line, one would have to adorn \mathbb[R} with an overbar, tilde, or asterisk, etc.
    – murray
    Apr 14 '20 at 17:01
11

What about \leftfree \mkern -12mu \rightfree from MnSymbol package?

enter image description here

\documentclass{report}
\usepackage{amssymb}
\usepackage{MnSymbol}
\begin{document}
\[ \mathbb{R}_{\leftfree\mkern -12mu\rightfree} \]
\end{document}
4
  • Thanks a lot! It works. I would also like to know how to type the symbol $\mathbb{R}_{[-]}$ as well. Apr 12 '20 at 15:07
  • @Howardli621, I'm looking for such squared symbol but maybe there is no one.
    – Sigur
    Apr 12 '20 at 15:20
  • 10
    @Sigur Usual caveat: using MnSymbol changes all mathematical symbols.
    – egreg
    Apr 12 '20 at 16:51
  • @egreg, yes, well remembered. I don't know how to load a single glyph but I know it is possible... and I know you know it :-)
    – Sigur
    Apr 12 '20 at 17:00
27

Sigur's answer might be appealing, but it has a big defect: with \usepackage{MnSymbol} you change every math symbol in your document to shapes that are thought to accompany Minion.

Besides, I find the strokes in MnSymbol to be too heavy. Your symbol should be as less intrusive as possible. How to use MnSymbol without all these defects?

\documentclass{article}
\usepackage{amsmath,amssymb}

% from MnSymbol.sty
\DeclareFontFamily{U}{MnSymbolA}{}
\DeclareFontShape{U}{MnSymbolA}{m}{n}{
    <-6>  s*[0.7] MnSymbolA5
   <6-7>  s*[0.7] MnSymbolA6
   <7-8>  s*[0.7] MnSymbolA7
   <8-9>  s*[0.7] MnSymbolA8
   <9-10> s*[0.7] MnSymbolA9
  <10-12> s*[0.7] MnSymbolA10
  <12->   s*[0.7] MnSymbolA12}{}

\DeclareRobustCommand{\usual}{%
  \text{\usefont{U}{MnSymbolA}{m}{n}\ooalign{\char"80\cr\char"82\cr}}%
}

\begin{document}

$\mathbb{R}_{\usual}$

\end{document}

Explanation: MnSymbol tells us that the required symbols are from MnSyA that's defined as a math symbol font in the package itself. So it's easy to borrow the font definitions and adding a scale factor to make the glyph smaller.

The font table reveals that the two symbols are at positions "80 and "82 respectively, so we can use them in \ooalign that superimposes them inside \text, without wasting a math font family.

enter image description here

2
  • 2
    @Sigur \ooalign is a low level construct using alignments where rows must be terminated by \cr. This is essentially the same as \makebox[0pt][l]{\char"80}\char"82
    – egreg
    Apr 12 '20 at 17:28
  • Oh, thanks. Now it makes sense to me.
    – Sigur
    Apr 12 '20 at 17:29
15

Another possibility, cutting out the parenthesis from a \bigcirc to get more round symbols. To get a more similar symbols with square brackets, I cut them out of amssymb's \square.

\documentclass{article}
\usepackage{mathtools}
\usepackage{amssymb}
\usepackage{trimclip}
\newcommand{\leftparen}{\clipbox{0 0 5.5pt 0}{\(\scriptstyle\bigcirc\)}}
\newcommand{\rightparen}{\clipbox{5.5pt 0 0 0}{\(\scriptstyle\bigcirc\)}}
\newcommand{\leftsquare}{\raisebox{-.9pt}{\clipbox{0 0 4.5pt 0}{\(\scriptstyle\square\)}}}
\newcommand{\rightsquare}{\raisebox{-.9pt}{\clipbox{4.5pt 0 0 0}{\(\scriptstyle\square\)}}}
\newcommand{\leftrightopen}{\mathrlap{\leftparen}-\!\!-\mathllap{\rightparen}}
\newcommand{\leftrightclosed}{\mathrlap{\leftsquare}-\!\!-\mathllap{\rightsquare}}
\newcommand{\Rtopopen}{\mathbb{R}_{\leftrightopen}}
\newcommand{\Rtopoclosed}{\mathbb{R}_{\leftrightclosed}}
\begin{document}
\( \Rtopopen \qquad \Rtopoclosed \)
\end{document}
12

Just because a tikz answer is always required ... Here is one using the Arc Barb arrowhead style of the arrows.meta library.

EDIT Added a line cap=round option to show that you can pretty much customize the shape as you need.

\documentclass{article}
\usepackage{amssymb}
\usepackage{tikz}
\usetikzlibrary{arrows.meta}

\newcommand{\topo}{%
    {\tikz {
        \draw[<->,>={Arc Barb[length=0.15em,width=1ex]}] (0,0) -- (0.7em,0); 
    }}%
}
\newcommand{\toporound}{%
    {\tikz {
        \draw[<->,>={Arc Barb[length=0.15em,width=1ex,line cap=round]}] (0,0) -- (0.7em,0); 
    }}%
}

\begin{document}
    $ \mathbb{R}_\topo$ $ \mathbb{R}_\toporound$
\end{document}

enter image description here

7
  • 1
    Why are you using [0]?
    – Sigur
    Apr 12 '20 at 15:14
  • @Sigur Actually, I guess I got used to do it this way for macros that do not take any input argument. Is it particlarly wrong ?
    – BambOo
    Apr 12 '20 at 15:15
  • 1
    If there is no argument, simply don't write anything: \newcommand{\foo}{faa}.
    – Sigur
    Apr 12 '20 at 15:19
  • @Sigur Ok, but does it make any difference ?
    – BambOo
    Apr 12 '20 at 15:22
  • 1
    @Sigur Haha, I see... I will apply your advice next time !
    – BambOo
    Apr 12 '20 at 15:26
10

Here are two possibilities.

enter image description here

\documentclass{article} 
\usepackage{amssymb} % for \mathbb macro
\begin{document}
$\mathbb{R}_{( \mkern-6mu -\mkern-6mu )}$ or
$\mathbb{R}_{\scriptscriptstyle( \mkern-6mu -\mkern-6mu )}$
\end{document}
1
  • BTW, I taught a lecture course (not in mathematics!) at HKUST in early January...
    – Mico
    Apr 12 '20 at 14:42
4

My idea is very simple...but strangely my command \mousse works :-))) ...using xy package :-) and not with sharp arrows. Obviously you can increase the length of the vector with round brackets with the parameter .7pc.

\documentclass[a4paper,12pt]{article}
\usepackage{mathtools}
\usepackage{amssymb}
\usepackage[all,cmtip]{xy}
\newcommand{\mousse}{\mkern-11mu\xymatrix@C=.7pc{
\ar@{(-)}[r]& 
}}
\begin{document}
$\mathbb{R}_{\mousse}\!\! \simeq \mathbb{C}_{\mousse}$
\end{document}

enter image description here

PS: It is not perfect like the other answers.

ADDENDUM:

A different style using scalerel package with \ttop command a bit little - parameter .7 - \scaleobj{.7}{}:

\documentclass[a4paper,12pt]{article}
\usepackage{mathtools}
\usepackage{amssymb}
\usepackage[all,cmtip]{xy}
\usepackage{scalerel}

\newcommand{\ttop}{\mkern-8mu\scaleobj{.7}{\xymatrix@C=.7pc{
\ar@{(-)}[r]& 
}}}
\begin{document}
$\mathbb{R}_{\ttop}\!\! \simeq \mathbb{C}_{\ttop}$
\end{document}

enter image description here

3

Here is a relatively simple solution using mathllap and manual kerning. Please Be aware that the parenthesis characters have a varying line width.

\documentclass{standalone}
\usepackage{mathtools}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{relsize}
\begin{document}
    $\mathbb{R}_{\mathrlap{(}\mkern0.5mu \relbar \mkern-10mu \relbar \mkern0.5mu \mathllap{)}}$
    $\mathbb{R}_{\mathrlap{[}\mkern0.5mu \relbar \mkern-10mu \relbar \mkern0.5mu \mathllap{]}}$
\end{document}

It looks like this:

enter image description here

1
  • Very nice also this solution.
    – Sebastiano
    Apr 15 '20 at 17:55

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