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When I use

\newcommand{\F1}{$\mathrm{F}_1$}

I get the Missing \begin{document}. error and missing $ inserted if I move it inside the document.

How do I define a macro so that \F1 would expand to $\mathrm{F}_1$?

I can do 

\DeclareMathOperator{\F1}{F_1}

but then I have to use it as $\F1$ instead of just \F1.

sds
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    As a general rule, macros can't have numbers in them. (Also, welcome to TeX.SX!) – Sean Allred Feb 20 '14 at 18:09
  • @HeikoOberdiek: no, this is a different issue! – sds Feb 20 '14 at 18:12
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    Try defining it like \newcommand\F[1]{$\mathrm{F}_#1$}. That will define a small macro \F that looks at the next 'token' at puts it as the subscript. (Keep in mind that one-letter macros are generally bad to make) – Sean Allred Feb 20 '14 at 18:13
  • @sds: Defining \H0, \H1, ... looks pretty close to \F1, doesn't it? – Heiko Oberdiek Feb 20 '14 at 18:14
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    @sds Unfortunately, it's not a different issue. Use something similar to the command definition I posted above if you want this kind of functionality. (Reading the TeXbook or TeX by Topic would be good reading for you concerning this.) – Sean Allred Feb 20 '14 at 18:15
  • @HeikoOberdiek: yes, sorry, this is, indeed, a dupe. – sds Feb 20 '14 at 18:17

1 Answers1

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Write \newcommand{\F}[1]{$\mathrm{F}_{#1}$}. This defines a macro which takes a single argument. You can then write \F1, which will expand to $\mathrm{F}_{1}$, \F2 which will expand to $\mathrm{F}_{2}$, etc. This form will work for any one letter argument, but you can also write \F{i,j} as short hand for $\mathrm{F}_{i,j}$.

Yossi Gil
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    Please, please, please use something more descriptive than \F in your actual document, though! – Sean Allred Feb 20 '14 at 18:18
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    What can be more descriptive to the letter 'F' than the macro \F? ;-) – Yossi Gil Feb 20 '14 at 18:19
  • @SeanAllred: F1 is quite idiomatic. – sds Feb 20 '14 at 18:19
  • @sds seems like \newcommand*\Fscore{\mathrm{F}_1} would be a good definition (the macro doesn't describe the letter F but a mathematical function). Also since it is most likely to be used in math-mode the definition should not contain $. $\Fscore=\frac{...}{...}$ – cgnieder Feb 20 '14 at 18:25
  • @srs That may be, but consider the shortcut I see all-too-often: \let\>\geq. One-letter (or excessively short) macro names are almost always used elsewhere. Redefine it and you risk breaking things in very bad, very unexpected ways. – Sean Allred Feb 20 '14 at 19:49