6

I use the bm package.

This is the expression I use:

\bm{A=\sqrt{B^2+C^2}}

My letters are bold as expected, but the square root is thinner and more ugly than without \bm.

Everything looks fine when using 

\bm{A=\sqrt{C}}

So I guess it has something to do with space.

Does somebody know how to fix this?

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    are you loading amsmath? If yes, you need to put the \sqrt{} macro in braces, like this: $\bm{A={\sqrt{B^2+C^2}}}$. Then it'll work. See page 9 of the bm manual. – Nico Vecchio Dec 11 '13 at 11:17
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    @dcmst page 9? actually \sqrt seems to need {} whether or not amsmath loaded, I'm sure it didn't used to – David Carlisle Dec 11 '13 at 12:16
  • @DavidCarlisle yup, you're right, it needs {} in both cases. I tried only the working cases without amsmath (like \sqrt{C}) and they obviously worked. This, plus I think I misread the referenced page. – Nico Vecchio Dec 11 '13 at 12:28
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    @dcmst see my answer :( – David Carlisle Dec 11 '13 at 13:02
5

Ah.

This can not possibly be a bug, so it must be a (only slightly) documented feature.

As mentioned in the bm manual, commands with optional arguments are locally modified inside \bm specifically so that \bm{\sqrt{xyz}} works. To see why you need \bm{{\sqrt{...}}} with an extra pair of braces in your example, consider:

enter image description here

\documentclass{article}

\usepackage{bm}
\showoutput
\begin{document}

$\sqrt{1x}$ 

$\sqrt{2x^2}$

$\sqrt{3x\strut^2}$

$\bm{\sqrt{4x^2}}$

$\bm{\sqrt{5x}}$

$\bm{{\sqrt{6x^2}}}$

\boldmath

$\sqrt{7x^2}$

$\sqrt{8x\strut^2}$

\end{document}

Looking at the relevant bit of the log file

$ grep \\\\OM[SX] bm111.log 
......\OMS/cmsy/m/n/10 p
......\OMS/cmsy/m/n/10 p
......\OMX/cmex/m/n/5 q
......\OMS/cmsy/b/n/10 p
......\OMS/cmsy/b/n/10 p
.........\OMS/cmsy/b/n/10 p
......\OMS/cmsy/b/n/10 p
......\OMX/cmex/m/n/5 q

You can see that "small" radicals come from the symbol font OMS/cmsy/m which has a bold version installed OMS/cmsy/b but large radicals come from the symbol extension font OMX/cmex/m which in the standard Computer Modern font set doesn't have a bold version. Your case with a superscripted capital is on a tipping point, if bm sets it all together (due to the extra braces) the cramped style is used, the radical gets chosen from cmsy and goes bold, but without the extra help the radical comes from cmex for which there is no bold font.

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