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I'm typing a bold vector small x with a tilde sign above it. Now when I want to write its derivative, I'm putting a dot on top of everything. This is giving me problems and I found one solution:

\overset{\mathrm{\textbf .}}{\boldsymbol{\mathrm{\tilde{x}}}}

after I read this post: http://tex.stackexchange.com/questions/1874/how-to-make-a-larger-dot . This post has a solution for a similar problem: \overset{\bm .}{a}

To write bold mathematical symbols, such as vectors is \boldsymbol better or do you recommend using the \bm package? I have always been using \boldsymbol.

Thanks a lot...

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2 Answers

up vote 12 down vote accepted

If you take a look at the documentation for bm, it says that if the bold fonts are not allocated, it will use the correct symbol—if it exists—by using \boldmath, but that this is slower. If there is not a bold version of the symbol, it uses poor man's bold.

I think that in most cases, you want to use \bm.

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Thanks. If bm is slower, then I guess sticking to \boldsymbol may be a better choice. Using \boldsymbol I guess will be better than a poor man's bold? Is this correct? –  yCalleecharan Sep 19 '10 at 9:46
    
@yCalleecharan: No, you misunderstood. As I understand the documentation, \bm will use the appropriate symbol if the symbol exists in the font and it's loaded. If the symbol exists in a font that isn't loaded, then it will still be used via \boldmath which is slower than the previous case. If there is no symbol, then it uses poor man's bold. –  TH. Sep 19 '10 at 10:05
    
Ok thanks. I only use the computer modern fonts in TeXLive. I will take a better look at the bm documentation. –  yCalleecharan Sep 19 '10 at 12:33
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The \bm command from package bm has both advantages and drawbacks over the \boldsymbol command from package amsbsy (loaded by amsmath). The first advantage of \bm is that it keeps the italic correction, so that something like \bm{T}_1^2 will look better than \boldsymbol{T}_1^2 (if you want to compare the two in a document, you must be careful that bm redefines \boldsymbol):

bm vs. boldsymbol: italic correction

The second advantage is that \bm does not disrupt the spacing, whereas \boldsymbol does, even though it works correctly for binary operations (like +) and for relations (like =). As an example, here's the comparison between \boldsymbol{[}-1;1] and \bm{[}-1;1]:

bm vs. boldsymbol: spacing

However, \bm sometimes requires the user to add braces to avoid problems. For example, \bm{\dots} will provoke an error with amsmath, the solution being to type \bm{{\dots}}. Double accents also need to be enclosed in braces to work properly:

\documentclass{article}
\usepackage{amsmath,bm}
\begin{document}
%$\bm{\dots}$% causes an error
$\bm{\mathinner{\dots}}$% works
$\bm{\hat{\dot{\phi}}}$% does not work correctly
$\bm{{\hat{\dot{\phi}}}}$% works correctly
\end{document}

bm double accents: with and without braces

Finally, math alphabet nesting works better with \boldsymbol than with \bm. For example, here's the result of \mathrm{g\boldsymbol{g}} and \mathrm{g\bm{g}}:

bm vs. boldsymbol: font nesting

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@Philippe: instead of \boldsymbol{T}_1^2 one can use \boldmath${T}_1^2$\unboldmath instead of \bm –  Herbert Feb 8 '11 at 22:47
    
@Herbert: Yes, but that will also put the 1 and the 2 in bold (which may not be wanted) and will not work if you want it to appear inside a longer formula, e.g. $C+\int_a^b\bm{T}_1^2(t)dt$, where nothing else is in bold. –  Philippe Goutet Feb 9 '11 at 9:20
    
@Philippe: I have everything in bold here: \boldmath$C+\bm{\int}_a^bT_1^2(t)dt$ –  Herbert Feb 9 '11 at 9:55
1  
@Herbert: if you want to put all the formula in bold, using \boldmath is fine, but if you want only one of the symbols of the formula to be in bold, you must use \bm or \boldsymbol (or \mathbf) on it and not touch anything else. It's in that case that the lack of italic correction is problematic with \boldsymbol. –  Philippe Goutet Feb 9 '11 at 12:56
2  
This excellent comparison is underrated and should be the accepted answer. I've bumped it by some minor grammar fixes. –  Hendrik Vogt Feb 21 '11 at 14:04
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