# How do I make mu bold and not the subscript?

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:

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


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

• 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
• 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

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.