How to Bold Greek Letters in LaTeX
Overview
The command used to represent vectors and the like in $\TeX$ is \mathbf; for example, \mathbf{x} is rendered as $\mathbf{x}$. However, this command does not work on Greek letters: \mathbf{\alpha} is still rendered as $\mathbf{\alpha}$, which looks identical to $\alpha$, the rendering of the basic \alpha.
Code
| $\TeX$ | Rendering |
|---|---|
f | $f$ |
\mathbf{f} | $\mathbf{f}$ |
\boldsymbol{f} | $\boldsymbol{f}$ |
\alpha | $\alpha$ |
\mathbf{\alpha} | $\mathbf{\alpha}$ |
\boldsymbol{\alpha} | $\boldsymbol{\alpha}$ |
For reference, as you can see, unlike \mathbf which works on the alphabet but not on Greek letters, \boldsymbol works on both the alphabet and Greek letters. Meanwhile, you can see that although \mathbf{f} $\mathbf{f}$ and \boldsymbol{f} $\boldsymbol{f}$ are both bold in weight, their appearance differs, and this is because the functions of \mathbf and \boldsymbol are actually different.
\mathbf, put simply, renders the ‘bold version’ of a given character that is widely used in mathematics. Greek letters do not have a separate bold typeface, which is why it does not work on them.\boldsymbol, in a sense, corresponds to the bold that exists in our common sense. It works simply by increasing the weight of the font.