I have a diagram in which I need to indicate the difference between symbolic mathematics and English words (potentially to someone who doesn't know about it):
Colour works beautifully for making this distinction. Unfortunately, for the version that is going to go in a book, I can't use colour and have been told not to use greyscale (which was my first fallback). I've tried bold, but it looks pretty horrible and doesn't highlight the distinction terribly well:
(Also bold is bad because sometimes it has mathematical significance.)
Underlining just looks really horrible:
I would be very grateful for any alternative suggestions on how to visually highlight symbolic material.
Source in case it's useful:
\documentclass{article}
\usepackage{xcolor}
\usepackage{framed}
\begin{document}
\definecolor{textual-colour}{rgb}{0,0,0}
\definecolor{symbolic-colour}{rgb}{0.2,0.2,0.9}
\newcommand{\marktextual}[1]{{\color{textual-colour}#1}}
\newcommand{\marksymbolic}[1]{{\color{symbolic-colour}#1}}
\begin{framed}
\marktextual{{If} \marksymbolic{$K \leq G$} {and} {there} {are} {inclusions} \marksymbolic{$gKg^{-1}\leq K$} {for} {every} \marksymbolic{$g\in G$}, {then} \marksymbolic{$K \triangleleft G$}: {replacing} \marksymbolic{$g$} {by} \marksymbolic{$g^{-1}$}, {we} {have} {the} {inclusion} \marksymbolic{$g^{-1}Kg\leq K$}, {and} {this} {gives} {the} {reverse} {inclusion} \marksymbolic{$K\leq gKg^{-1}$}.}
\smallskip
\marktextual{{The} {kernel} \marksymbolic{$K$} {of} {a} {homomorphism} \marksymbolic{$f:G\rightarrow H$} {is} {a} {normal} {subgroup}: {if} \marksymbolic{$a\in K$}, {then} \marksymbolic{$f(a)=1$}; {if} \marksymbolic{$g\in G$}, {then} \marksymbolic{$f(gag^{-1}) = f(g)f(a)f(g^{-1}) = f(g)f(g^{-1}) = 1$}, {and} {so} \marksymbolic{$gag^{-1}\in K$}. {Hence}, \marksymbolic{$gKg^{-1}\leq K$} {for} {all} \marksymbolic{$g\in G$}, {and} {so} \marksymbolic{$K \triangleleft G$}. {Conversely}, {we} {shall} {see} {later} {that} {every} {normal} {subgroup} {is} {the} {kernel} {of} {some} {homomorphism}.}
\end{framed}\end{document}
Edit: while I was trying to keep this short, from the comments it looks like a little more explanation of the context is necessary. The text of the book says:
"At first sight, the most striking feature of mathematical language is the way in which it mixes material that looks as if it is drawn from a natural language with material built up out of idiosyncratically mathematical symbols. The distinction is illustrated in Figure 2.1."
Figure 2.1 looks like this: