All letters upright in math mode (equivalent to \rm)

Question

I'm using the document class scrartcl and I'm looking for a way to write equations in which all letters are upright like it appears when using \rm.

\begin{equation}
    \rm A_{something} + B_{another} = C_{whatever}
\end{equation}

Because the use of \rm is deprecated my question is: is there way to achieve the same without enclosing every bit of text in \text{}?

\begin{equation}
    \text{A}_{\text{something}} + \text{B}_{\text{another}} = \text{C}_{\text{whatever}}
\end{equation}

Answer 1

If you like the effect of \rm but don't like that it is deprecated, then taking a look its implementation gives a good clue of what you can do. In source2e you will find (see p.254 of source2e 2017-04-15, where \DeclareOldFontCommand is defined)

 \DeclareOldFontCommand{\rm}{\normalfont\rmfamily}{\mathrm}

which means it is just \mathrm in math mode. Thus you could write \mathrm around your equation contents instead.

\documentclass{scrartcl}

\begin{document}

\begin{equation}
    \mathrm{A_{something} + B_{another} = C_{whatever}}
\end{equation}

\end{document}

Answer 2

If what you are after is an easy way of writing \text in subscripts, here's a way. You write A_[something] if you want \text{..} to be applied to the subscript.

\documentclass{scrartcl}

\usepackage{mathtools}

\def\sbtext[#1]{\sb{\textnormal{#1}}}
\def\sptext[#1]{\sp{\textnormal{#1}}}
\makeatletter
\begingroup\lccode`\~=`\_\lowercase{\endgroup
  \def~}{\@ifnextchar[\sbtext\sb}
\begingroup\lccode`\~=`\^\lowercase{\endgroup
  \def~}{\@ifnextchar[\sptext\sp}
\makeatother

\AtBeginDocument{\catcode`\_=12 \mathcode`\_="8000 \catcode`\^=12 \mathcode`\^="8000 }

\begin{document}

\begin{equation}
  A_[something] + B_[another] = C_[whatever]
\end{equation}

\end{document}

Also if one prefers A_|something| + B^|something| rather than A_[something] + B^[something] just change the definitions of \sptext|#1| and \sbtext|#1| and subsitute \@ifnextchar|.

Answer 3

It's not clear why you would do this; however, here it is.

\documentclass{article}
\usepackage{amsmath}

\newenvironment{uprightmath}
 {\changecodes\ignorespaces}
 {\ignorespacesafterend}

\newcommand{\changecodes}{%
  \count255=`A
  \loop
  \mathcode\count255=\numexpr\mathcode\count255-\string"100\relax
  \ifnum\count255<`Z
    \advance\count255 1
  \repeat
  \count255=`a
  \loop
  \mathcode\count255=\numexpr\mathcode\count255-\string"100\relax
  \ifnum\count255<`z
    \advance\count255 1
  \repeat
}

\begin{document}
Here the letters are normal
\begin{equation}
A_{x}+B_{y}=Z
\end{equation}
\begin{uprightmath}
But here they're upright $A_{something} + B_{another} = Z_{whatever}$
\begin{equation}
A_{something} + B_{another} = Z_{whateverz}
\end{equation}
and back to normal
\end{uprightmath}
\begin{equation}
A_{x}+B_{y}=C
\end{equation}
\end{document}

Answer 4

The question asks how to write equations, so this is not an answer in that sense. Here, I suggest a method on how to draw them.

Since + is an operation of coalescence, in general, and = indicates identity (such that C is A and B combined together), and since everything algebraic can be represented visually, the TikZ solution

is presented by:

MWE

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{automata,positioning}

\begin{document}
  \begin{tikzpicture}[%
    >=stealth,
    node distance=1.6cm,
    on grid,
    auto
  ]
    \node[state] (A)  [align=center,fill=blue!5]{{\large\textit{A}}};
    \node        (A1) [align=center,pos=0.25,left = of A,fill=green!15]{Raindrop\textit{\textsubscript{s\textsubscript{1}}}};
    \node        (S1) [below = of A]{};
    \node[state] (B)  [align=center,fill=blue!5,below=of S1]{{\large\textit{B}}};    
    \node        (B1) [align=center,left = of B,fill=green!15]{Raindrop\textit{\textsubscript{s\textsubscript{2}}}};
    \node[state,double] (C)  [align=center,fill=blue!5,right=of S1,minimum size=3.8em]{{\large\textit{AB}}};    
    \node        (C1) [align=center,right = of C,fill=green!15]{Raindrop\textit{\textsubscript{B\textsubscript{1}}}};
    \draw[->]   (A) edge (C);
    \draw[->]   (B) edge (C);    
      \end{tikzpicture}
\end{document}

which may be easier for some to grasp, since mixing two styles (algebra and plain language) adds a cognitive load for the reader since they have to untangle the two and then compare how their meanings intertwine.

It becomes immediately apparent upon inspection that the AB combination, C, has properties different to its constituents (e.g., evenness), implying that the identity (=) is only a partial description. A transformation has also occurred, with the creation of a new entity.

The letters can be upright or italic, as one pleases.