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I'm trying to match this:

enter image description here

I've already tried (in amsmath)

$A^0$
$A^o$
$A^\circ$

None of these match the above image however.

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2  
Welcome to TeX.SX! It could be $A^{\mathrm{o}}$. Can you add a source for such a notation? – egreg Feb 24 at 0:01
    
Unfortunately with that the circle is too low, but definitely matches it better than the others (but not perfectly). As for a source, the image was taken from a scan. (I would like to confirm that I am user99133 but had somehow managed to post under a guest account) – Irregular User Feb 24 at 0:05
1  
font differences are to be expected, but what is the intended meaning, is that an index 0 or a superscipt O when taken in context? – David Carlisle Feb 24 at 0:07
    
It's the interior of the set A, usually seen in topology. The index is much closer to an o rather than a 0. As for font differences, I understand that but would like to match it as close as possible. – Irregular User Feb 24 at 0:10
    
wikipedia suggests that it is ^o with a lowercase o en.wikipedia.org/wiki/Interior_%28topology%29 – David Carlisle Feb 24 at 0:10
up vote 5 down vote accepted

The symbol seems to be an upright “o”; in order to raise it more than it would be with $A^{\mathrm{o}}$, you can define a macro.

\documentclass{article}

\newcommand{\interior}[1]{%
  {\kern0pt#1}^{\mathrm{o}}%
}

\begin{document}

$\interior{A} \interior{B}$

\end{document}

enter image description here

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This is perfect, thank you very much! I would choose this as the answer if I could but I have no idea how to get back into the guest account. So instead, take an upvote. – Irregular User Feb 24 at 0:25
1  
@IrregularUser See stackoverflow.com/help/merging-accounts – egreg Feb 24 at 10:11
    
@egreg. Why has the 0pt kern the effect of raising the exponent? I can't understand. – User Mar 2 at 17:56
1  
@User In this case TeX ignores the metric information of A and just looks at the height of the box, because the nucleus of the math atom is not a single math character. – egreg Mar 2 at 18:37
    
@egreg. Thank you for the explanation – User Mar 2 at 19:09

I'd use the first, but take your pick:-)

enter image description here

\documentclass{article}

\begin{document}


\[
A^\mathrm{o}
\quad
A\strut^\mathrm{o}
\quad
A\mkern-1mu\vrule width0pt height 1em^\mathrm{o}
\quad
A\mkern-1mu{\vrule width0pt height 2ex}^\mathrm{o}
\]

\end{document}
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Above all, use a macro, that way you can change it later (or even provide it with a few intelligence), here's a basic version

\newcommand*\interior[1]{#1^{\mathsf{o}}}

You can let \interior be intelligent, and do (#1)^{\mathsf{o}} in case there are a few symbols inside, or even some parenthesis above the whole expression like some notations do.

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The latex kernel contains the \mathring accent for that:

enter image description here

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