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Suppose I want to have the function $\bar: A \to B$ where instead of $\bar$ it's a longer bar. What is the proper way to do this? Note that the bar here is floating on top of no text. It's just floating.

I also wasn't sure what tags to use. Sorry.

Thanks.

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    I wouldn't go for that notation. It would be clearer to write it out: We define $x\to \overline{x}$ as a map $A\mapsto B$. Or something like that, I'm not a very good English speaker. If you go for the direct notation anyways, use parentheses to clarify the notation: $(\overline{\phantom{A}}):A\mapsto B$.
    – yo'
    Jul 21, 2014 at 19:19
  • Not what you're asking about but $f\colon A\to B$ puts the colon in the right place: something like f: A -> B instead of f : A -> B.
    – David Richerby
    Jul 21, 2014 at 21:24

2 Answers 2

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If I understand what you're trying to do, you can use \overline{\phantom{<text>}}:

\documentclass{article}
\pagestyle{empty}
\begin{document}

$\overline{\phantom{A}}: A\to B$

$\overline{\phantom{AA}}: A\to B$

$\overline{\phantom{AAA}}: A\to B$

\end{document}
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If I may, there is another alternative that provides more flexibility. i.e., Use of 

\rule[raise-height]{width}{thickness}

The \rule command is used to produce horizontal lines. The arguments are defined as follows.

raise-height specifies how high to raise the rule (optional)

width specifies the length of the rule (mandatory)

thickness specifies the thickness of the rule (mandatory)

enter image description here

Code

\documentclass{article}
\usepackage[paper size={12cm,8cm}
\pagestyle{empty}
\begin{document}


baseline + 0cm \hfil shorter \hfil thicker

\hfil $\rule[0cm]{1cm}   {0.1pt}: A\to B$  
\hfil $\rule[0cm]{0.5cm} {0.1pt}: A\to B$  
\hfil $\rule[0cm]{1cm}   {1pt}:   A\to B$  \hfil

\medskip    baseline + 0.1cm    \medskip

\hfil $\rule[0.1cm]{1cm}  {0.1pt}:  A\to B$ 
\hfil $\rule[0.1cm]{0.5cm}{0.1pt}:  A\to B$ 
\hfil $\rule[0.1cm]{1cm}  {1pt}:    A\to B$ \hfil

\medskip    baseline + 0.3cm     \medskip

\hfil $\rule[0.3cm]{1cm}   {0.1pt}: A\to B$ 
\hfil $\rule[0.3cm]{0.5cm}{0.1pt}:  A\to B$ 
\hfil $\rule[0.3cm]{1cm}  {1pt}:    A\to B$ \hfil

\medskip    baseline + 0.5cm    \medskip

\hfil $\rule[0.5cm]{1cm}   {0.1pt}: A\to B$
\hfil $\rule[0.5cm]{0.5cm} {0.1pt}: A\to B$ 
\hfil $\rule[0.5cm]{1cm}   {1pt}:   A\to B$ \hfil

\end{document}
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