[Tex/LaTex] \mathit vs \textit for multi-letter identifiers

fontsmath-mode

I would like to include a number of two-letter variables in a sentence separated by commas but sometimes there is a noticeable space between the variable and the comma that follows it when I use mathit. Although the spaces are not as noticeable in the MWE below.

Is it better to use textit instead of mathit? If so, how can I ensure the font matches single-letter variables using the same letters (e.g. $\textit{A}$ )?

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\textit{AB}, \textit{BA}, \textit{CD}, \textit{DC}

$\mathit{AB}$, $\mathit{BA}$, $\mathit{CD}$, $\mathit{DC}$

$\textit{AB}$, $\textit{BA}$, $\textit{CD}$, $\textit{DC}$
\end{document}

Best Answer

This is not an answer, but an attempt to talk you out of this notation.

UPDATE This post got feedback from several more experienced users. I agree with all their statements and certainly should have done more research. However, I also feel that their statements provide further reasons for the proposed notation not being optimal.

\documentclass{article}
\usepackage{amsmath}
\newcommand\myvar[1]{\ensuremath{\mathit{#1}}}
\begin{document}
Your options 
\begin{itemize}
 \item \textit{AB}, \textit{BA}, \textit{CD}, \textit{DC}
 \item $\mathit{AB}$, $\mathit{BA}$, $\mathit{CD}$, $\mathit{DC}$
 \item $\textit{AB}$, $\textit{BA}$, $\textit{CD}$, $\textit{DC}$
\end{itemize}
look all the same. In fact, it seems that there is no noticable difference
between these options:\\
\verb|\makebox[0pt][l]{$\mathit{AB}$}$\textit{AB}$|: \makebox[0pt][l]{$\mathit{AB}$}$\textit{AB}$

However, things are different once you wrap these commands in other
environmants:\\
\verb|\textbf{$\mathit{AB}$}|: \textbf{$\mathit{AB}$} vs.\\
\verb|\textbf{$\textit{AB}$}|: \textbf{$\textit{AB}$}\\
\verb|$\left(\mathit{AB}\right)$|: $\left(\mathit{AB}\right)$ vs.\\
\verb|$\left(\textit{AB}\right)$|: $\left(\textit{AB}\right)$

Even the pure math version looks very similar:\\
\verb|\makebox[0pt][l]{$\mathit{AB}$}$AB$|: \makebox[0pt][l]{$\mathit{AB}$}$AB$ 

However, the pure math version comes with different distances:\\
\verb|\makebox[0pt][l]{$\mathit{BA}$}$BA$|: \makebox[0pt][l]{$\mathit{BA}$}$BA$\\
\verb|\makebox[0pt][l]{$\mathit{BA}$}$\textit{BA}$|: \makebox[0pt][l]{$\mathit{BA}$}$\textit{BA}$

You may come up with a command that distinguishes netween math and nonmath
modes: \verb|\newcommand\myvar[1]{\ensuremath{\mathit{#1}}}|

Under boldface, this seems to behave like a math variable should:\\
\verb|\textbf{\myvar{AB}}|: \textbf{\myvar{AB}}\\
\verb|\textbf{$\myvar{AB}$}|: \textbf{$\myvar{AB}$}\\
\verb|\boldmath$\myvar{AB}$\unboldmath|: \boldmath$\myvar{AB}$\unboldmath\\
\verb|\boldmath\myvar{AB}\unboldmath|: \boldmath\myvar{AB}\unboldmath

The main reason for this point is to question whether it is a very wise move to
use this notation. Many people use $AB$ to indicate the
product $A\cdot B$, and $AB$ is virtually indistinguishable from your variable
\myvar{AB}. (I personally write $A\,B$, but get sometimes criticized.)

\paragraph{Bottomline:} Perhaps it is better to use a different notation.
\end{document}

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[Tex/LaTex] Multiple letters without spacing in Math

It's theoretically possible to make LaTeX recognize clusters of letters and typeset them as if they were input as argument to \mathit, but it would be deadly slow: each alphabetic character should be turned into a “math active” one, which checks whether the following item is an alphabetic character; if it is it should typeset itself, starting \mathit if it's the first one, and then pass the same control to the next character; if not followed by an alphabetic character it should end \mathit.

It's instead better to do something like

\newcommand{\mli}[1]{\mathit{#1}}

where \mli stands for MultiLetter Identifier; use any control sequence name. Then you'd input

$\mli{P}=\mli{NP}$

that has a good deal of advantages, but mainly keeps information about the input.

_{Note: I have a nice proof of the above statement, but unfortunately there are length limitations for posts on this site.}

Heiko's solution in expl3.

\documentclass[fleqn]{article}
\usepackage{xcolor}
\usepackage{amsmath}
\usepackage{xparse}

\ExplSyntaxOn

% 1. Define an equivalent for each letter
\cs_new_protected:Nn \__egreg_letter_loop:nn
 {
  \int_step_inline:nnn { `#1 } { `#2 }
   {
    \exp_args:Nc \mathchardef 
     { __egreg_letter_code_##1: } % generate the old code
     =
     \mathcode##1 % the old code
     \scan_stop:
    \cs_new_protected:cx { __egreg_letter_act_##1: }
     {
      \exp_not:N \__egreg_letter_scan:nw { ##1 }
     }
    \char_set_active_eq:nc { ##1 } { __egreg_letter_act_##1: }
   }
 }
\__egreg_letter_loop:nn { A } { Z }
\__egreg_letter_loop:nn { a } { z }

% 2. Define the main macro that does the scanning
\tl_new:N \l__egreg_letter_scanned_tl

\cs_new_protected:Npn \__egreg_letter_scan:nw #1
 {
  \tl_if_empty:NT \l__egreg_letter_scanned_tl { \c_group_begin_token } % to be closed later
  \tl_put_right:Nx \l__egreg_letter_scanned_tl { \exp_not:c { __egreg_letter_code_#1: } }
  \peek_catcode:NF A
   {% next char is not a letter
    \__egreg_letter_deliver:V \l__egreg_letter_scanned_tl
   }
 }
\cs_new_protected:Nn \__egreg_letter_deliver:n
 {
  \tl_if_single:nTF { #1 }
   {
    \__egreg_letter_single:n { #1 }
   }
   {
    \__egreg_letter_group:n { #1 }
   }
  \c_group_end_token % finish the group
 }
\cs_generate_variant:Nn \__egreg_letter_deliver:n { V }

% 3. The interface for defining the actions
\NewDocumentCommand{\definelettersingle}{m}
 {
  \cs_set_protected:Nn \__egreg_letter_single:n { { #1 } }
 }
\NewDocumentCommand{\definelettergroup}{m}
 {
  \cs_set_protected:Nn \__egreg_letter_group:n { { #1 } }
 }

% 4. Make all letters math active
\int_step_inline:nnn { `A } { `Z } { \mathcode#1="8000 }
\int_step_inline:nnn { `a } { `z } { \mathcode#1="8000 }

\ExplSyntaxOff

% initialize

\definelettersingle{#1}
\definelettergroup{\mathit{#1}}

\begin{document}

First a standard formula $NP\ne N P$

\bigskip

Now \textcolor{blue}{single letters are set in blue},
\textcolor{red}{multiple letters in red}.
\begin{itemize}
\item Multiple letters are put in \verb|\mathit|:
  \definelettersingle{{\color{blue}#1}}%
  \definelettergroup{\mathit{\color{red}#1}}%
  \[ NP \neq N P \]

\item Multiple letters are put in \verb|\mathrm|:
  \definelettersingle{{\color{blue}#1}}%
  \definelettergroup{\mathrm{\color{red}#1}}%
  \[ NP\ (text) \neq N P\ (two\ variables) \]
  \[ F_force = m_mass a_acceleration \]
\end{itemize}
\end{document}

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