Bug in using self-defined font?

Question

As long as I do not use \bf, my font works fine. Starting with $$ within which I first use \bf, and every instance thereafter, instead of getting the font I defined I keep getting bold letters. How do I fix this?

The problem might lie in the fact that I defined my own font. AMSFonts User Guide mentions several font families which can be used to define new fonts. Since it does not tell how to do so, I had to hack together a way to define my own font which I called \Cal.

\documentclass{article}
\usepackage{amsfonts}

\newfam\calfam
\font\calten=eusm10
\font\calseven=eusm7
\font\calfive=eusm5
\textfont\calfam=\calten
\scriptfont\calfam=\calseven
\scriptscriptfont\calfam=\calfive
\def\Cal{\fam=\calfam}

\begin{document}
${\Cal O}$ ${\Cal O}{\bf a}b{\Cal O}$ ${\Cal O}$
\end{document}

The first \Cal O shows up fine; the rest show up bold instead. Note that the b does NOT show up bold: so the bold-ness has obviously ended with the a. However, the O's keep showing up bold thereafter. How can I get regular \Cal O's throughout my document?

Thank you in advance. Update:

I found the answer: Does it matter if I use \textit or \it, \bfseries or \bf, etc

{\bf foo}, for example, resets all font attributes which had been set earlier before it prints foo in bold face.

In other words, when I used bf, I guess it reset some variable that I had not defined when defining my font. While it would be nice to know how to define my font to allow it to go back to normal, I guess I will have to use the longer \mathbf in the meanwhile.

Answer 1

This is a typical “don't do it” code. The instructions \newfam and \font should never be used in LaTeX at the user's level. For instance, if you change the point size option of your document from the default 10pt to 11pt, you'll have to manually modify the font assignments.

You should rather rely on the font selection scheme, which is not really difficult. First, define a font family

\DeclareFontFamily{U}{eus}{\skewchar\font'60}
\DeclareFontShape{U}{eus}{m}{n}{%
     <-6>eusm5%
    <6-8>eusm7%
    <8->eusm10%
}{}
\DeclareFontShape{U}{eus}{b}{n}{%
     <-6>eusb5%
    <6-8>eusb7%
    <8->eusb10%
}{}

(the U encoding is predefined, and it applies no “transformation”). Then you can define a new math alphabet

\DeclareMathAlphabet{\matheucal}{U}{eus}{m}{n}
\SetMathAlphabet{\matheucal}{bold}{U}{eus}{b}{n}

In the document you'll use the “correct” syntax

$\matheucal{O}$ $\matheucal{O}\mathbf{a}b\matheucal{O}$ $\matheucal{O}$

With the \SetMathAlphabet instruction, the math alphabet will respect the \boldmath declaration and, if \usepackage{bm} is given, the code

\bm{\matheucal{O}}

will produce a boldface calligraphic “O” also under normal math version.

More than that, you'll ensure that the calligraphic letters have the correct size also in contexts where \large, \small, \footnotesize and similar declarations are in force, which will not be the case with your \font based approach.

This said, the eucal package does exactly what's outlined above; if you declare

\usepackage{eucal}

then Euler calligraphic will be output when \mathcal is used; if you want to keep the original meaning of \mathcal for using Euler calligraphic along with the default calligraphic (but you shouldn't, because the shapes are not so different from one another), load the package by

\usepackage[mathscr]{eucal}

that will make Euler calligraphic available with \mathscr.

Answer 2

\bf is deprecated in LaTeX, you should use \textbf instead if it's meant to be upright, or \bm with \usepackage{bm} for bold math if it's meant to be bold italic math.

Since you are using LaTeX, there's already a solution for your problem in the package eucal. With this you can avoid the "hack together" method entirely:

\documentclass{article}
\usepackage{amsfonts,eucal}
\usepackage{bm}

\begin{document}
${\mathcal O}$%
${\mathcal O}{\bm a}b{\mathcal O}$%
${\mathcal O}$
\end{document}