Typeset subscript material automatically in upright font shape

Question

As discussed for example here: Should subscripts in math mode be upright? subscripts should usually be typeset upright. Is there a possibility to this automatically e.g. inserting something to the document-header? Or, if not possible in LaTeX, is there an option in LyX? Until now I do it like this: Q _ Alt Z R test which is quite slow.

Answer 1

If you have a lot of subscripts, it might be helpful to put the following code in your preamble:

\makeatletter
 \begingroup
  \catcode`\_=\active
  \protected\gdef_{\@ifnextchar|\subtextup\sb}
 \endgroup
\def\subtextup|#1|{\sb{\textup{#1}}}
\AtBeginDocument{\catcode`\_=12 \mathcode`\_=32768}
\makeatother

Then you can write, say, $A_|p|$ and get p in upright text mode.

MWE

\documentclass{article}

\makeatletter
 \begingroup
  \catcode`\_=\active
  \protected\gdef_{\@ifnextchar|\subtextup\sb}
 \endgroup
\def\subtextup|#1|{\sb{\textup{#1}}}
\AtBeginDocument{\catcode`\_=12 \mathcode`\_=32768}
\makeatother

\begin{document}

$A_|p|$

\end{document}

Answer 2

Here is TeX based solution.

\def\subinrm#1{\sb{\rm#1}}
{\catcode`\_=13 \global\let_=\subinrm}
\mathcode`_="8000
\def\upsubscripts{\catcode`\_=12 } \def\normalsubscripts{\catcode`\_=8 }

\upsubscripts

$A_{lake}$, $\normalsubscripts \sum_{i=1}^\infty {1\over n}$

Answer 3

Here's a LuaLaTeX-based solution. The solution assumes that subscript material that's encased in curly braces should be rendered in the upright font shape. The main input syntax requirement is that there mustn't be whitespace between the _ and { characters. $R_{lake}$ works, but $R_ {lake}$ does not. (Whitespace inside the curly braces is OK, though.)

For the convenience of entering single-character subscripts -- a frequently occurring situation! -- it is further assumed that single-character subscripts that are not encased in curly braces should not be rendered in the upright font shape.

Finally, if there is subscript material which is encased in curly braces but should not be rendered with upright letters, you will either need to place the material into math mode explicitly, as is done in the fourth term shown below, or leave a gap between _ and {, as mentioned earlier. I.e., one would have to write $\sum_ {i=0}^\infty$.

% !TEX TS-program = lualatex
\documentclass{article}   
\usepackage{amsmath,luacode}
\begin{luacode}

function up_subs ( buff )
    return ( string.gsub ( buff , "_(%b{})" , "_{\\textnormal%1}" ) )
end

\end{luacode}
\AtBeginDocument{\directlua{luatexbase.add_to_callback(
    "process_input_buffer", up_subs, "up_subs")}}
\begin{document}

$R_t$ $R_{lake}$ $\sum_i$ $\sum_{$i{=}0$}^\infty$ % or: \sum_ {i=0}^\infty

\end{document}

Answer 4

As posted here, the answer above by wipet does not work with script classes like scrartcl, but can easily be updated to do so:

\documentclass{scrartcl}
\usepackage{amsmath}

% typesetting indexes upright by default
\def\subinrm#1{\sb{\textnormal{#1}}}
{\catcode`\_=13 \global\let_=\subinrm}
\mathcode`_="8000
\def\upsubscripts{\catcode`\_=12 } \def\normalsubscripts{\catcode`\_=8 }
% the toggle for upright subscripts
\upsubscripts
% the toggle for italic subscripts
%\normalsubscripts

\begin{document}
    $A_{lake}$, $\normalsubscripts \sum_{i=1}^\infty {1\over n}$
\end{document}

If you do not need accented characters, such as umlauts, you should use \mathrm instead of \textnormal, as David Carlisle pointed out. Then, the amsmath package is not strictly required either.