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I'm not sure how to do this and have spent a long time trying.

I looked at a previous post on this: Bold subscripts

It is slightly different but, I still can't get it to work.

My code:

\begin{equation} 
p(\textbf{x}) = \sum_{n=1}^{K} \pi_k \mathcal{N}
(\textbf{x}|\mathbf{\mu}_{k},\mathbf{\Sigma}_{k})
\end{equation}

enter image description here

As you can see, I can get the sigma in bold, but not the mu?

Thanks in advance

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    If you are using bold to represent something, for instance a vector, I would define a command \newcommand*\vector[1]{\mathbf{#1}} and that way you get clearer code an easy to change. In any case, for \mu you would be better using \bm from bm package. – Manuel Nov 30 '17 at 10:58
  • Welcome to TeX.SE. Your title is misleading: You don't want to capitalize but to get a bold \mu`. – user31729 Nov 30 '17 at 10:58
  • @Manuel Wow. \bm worked. Thanks so much! I spent nearly an hour on this LOL -.o – Tauseef Nov 30 '17 at 11:05
  • 1
    the \mathbf is not expected to do anything good on \mu in traditional pdflatex context. You can use also \boldsymbol{\mu} if using amsmath. Admittedly your \mathbf{\Sigma} works... turns out indeed that in the default setting \Sigma is of variable family type but not \mu. – user4686 Nov 30 '17 at 11:54
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I am adding an answer for a remark (the OP got the answer from @Manuel's comment to use \bm from bm package; one can also use \boldsymbol from amsmath package).

The remark here is that there was a problem with \mu but not with \Sigma. Why?

The point is that the cmr font contains 11 uppercase greek letters in slots 0 to 10, in the OT1 encoding. This is a bizarre legacy, but it does make \mathbf{\Sigma} work. I.e. the font used for operator names (log, sin, cos,...) also contains the uppercase (non-Latin) Greek letters. The idea of the \math.. alphabet commands is to use the text font in math mode. That it works for those 11 Greek letters is counter-intuitive and bound to the OT1 encoding used by the "operator" font. It is not robust against changes of the "operator" font.

Hence I would not recommend using \mathbf{\Sigma}, because it twists too much the idea of the LaTeX NFSS math alphabet commands. It ties it to peculiarity of Knuth's OT1 encoding.

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  • Yeah, although the problem was solved the mystery was not solved. Thanks for the explanation! – Tauseef Dec 2 '17 at 11:52

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