# Is there a convention for making text as an index in math

Taking an example from category theory, given a category "C", the opposite category is usually denoted "C^op". Where "op" is an abbreviation for opposite. Now there are three ways to put this in latex:

• As C^{op}
• As C^\operatorname{op}
• As C^\text{op}

While I wouldn't use the first one, thw second and third both make sense to me in some way. So is their a convention which one should use, and if so, what's the reasoning behind it

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The first one might not be a good idea, like you already noticed, because op could be mistaken for o⋅p. But I don’t think the other options are correct either.

\operatorname{op} puts correct spacing around keywords, when used as mathematical operators. That is not the case here.

\text{op}, at least syntactical, means prose. But you are trying to display mathematical notation.

So I think the best way is to use \mathrm{op}. Or still better, define

\newcommand\opcat[1]{{#1}^{\mathrm{op}}


and use it as \opcat{C}.

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7 seconds slower (but we agree on the answer:-) –  David Carlisle Feb 12 at 14:39
@DavidCarlisle: Indeed, congratulations, you have my upvote ;-) –  canaaerus Feb 12 at 14:40
Likewise, you have mine:-) –  David Carlisle Feb 12 at 14:44
I would argue for the use of \text, due to its font-preserving properties, and the fact that the label (not an index) is actually text. You're not merely marking up some mathematical symbols in a Roman font, you're indicating the presence of a standard vernacular word or abbreviation thereof. –  Xerxes Feb 12 at 18:36
@Xerxes I'd strongly disagree here op is not an abbreviation for a natural language vernacular word; it is a specific categorical operator which just happens to use a superscript rather than prefix notation. It should not pick up the surrounding text font any more than log or sin should. –  David Carlisle Feb 13 at 10:40
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Definitely not ^{op} the math italic font should never be used for multi-letter identifiers.

^\operatorname{op} (by default) uses the same font as \mathrm with \mathop spacing (the same as \log or \sin. The \mathop has no effect in the usage shown but would in other contexts.

^\text{op} (with the amsmath definition of \text) will make a mathord, using the current text font.

Probably I would use (a macro defined as) \mathrm{op} so that it uses the \mathrm font with mathord spacing.

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