In the example below, it is my feeling that \boldsymbol badly increases the horizontal spacing with the index:

\documentclass[fleqn,11pt]{book}
\usepackage{amsmath}
\begin{document}
$\boldsymbol{\sigma}_0$ $\sigma_0$
\end{document}


Shouldn't this be corrected?

Load the bm package and the problem goes away, both using \bm and \boldsymbol (which gets redefined by bm to be the same as \bm).

This spacing issue is one of the reasons why \bm should be used for bolding math instead of amsmath's \boldsymbol.

\documentclass[fleqn,11pt]{book}
\usepackage{amsmath,bm}
\begin{document}
\begin{align*}
\boldsymbol{\sigma}_0&\\
\bm{\sigma}_0&\\
\sigma_0&
\end{align*}
\end{document}


The difference can be seen with the following example

\documentclass[fleqn,11pt]{book}
\usepackage{amsmath}
\let\amsboldsymbol\boldsymbol
\usepackage{bm}
\begin{document}
\begin{tabular}{rr}
\verb|amsmath's \boldsymbol| & $\amsboldsymbol{\sigma}_0$ \\
\verb|\bm| & $\bm{\sigma}_0$ \\
& $\sigma_0$
\end{tabular}
\end{document}


• As you note, if bm and amsmath are both loaded, there's no difference in the outputs of \boldsymbol{\sigma} and \bm{\sigma}. The recommendation should thus focus on always loading the bm package, rather than on preferring \bm over \boldsymbol, right? – Mico Mar 25 '15 at 6:45
• @Mico In fact. bm does \let\boldsymbol\bm. – karlkoeller Mar 25 '15 at 6:48
• in fact, the documentation for amsmath recommends bm, although not as strongly as it might. (on the list for improvement.) – barbara beeton Mar 25 '15 at 13:48

Yes, there's a minor increase in the symbol width. Here is a correction using an \ooalign overlay:

\documentclass{article}
\usepackage{amsmath}
\newcommand{\boldsigma}{{\ooalign{\phantom{$\sigma$}\cr\hidewidth$\boldsymbol{\sigma}$\hidewidth}}}
\begin{document}

$\boldsymbol{\sigma}_0$ $\sigma_0$

Width of $\boldsymbol{\sigma}$: \savebox9{$\boldsymbol{\sigma}$}\the\wd9\par
Width of $\sigma$: \savebox9{$\sigma$}\the\wd9\par
Width of $\boldsigma$: \savebox9{$\boldsigma$}\the\wd9

$\boldsigma_0$ $\sigma_0$

\end{document}

• Your construction succeeds in making \boldsigma as wide as \sigma. However, \boldsigma_0 is about 0.36pt wider than \sigma_0. – Mico Mar 25 '15 at 5:26
• the problem isn't the width of the symbol but \boldsymbol makes an hbox so you lose the inter letter kerns that normally adjust subscript position and the subscript is placed after the box not tucked in to the italic letter. – David Carlisle Mar 25 '15 at 10:11