\boldmath & \unboldmath ¶Synopsis (used in paragraph mode or LR mode):
\boldmath \( math \)
or
\unboldmath \( math \)
Declarations to change the letters and symbols in math to be in
a bold font, or to countermand that and bring back the regular
(non-bold) default, respectively. They must be used when not in
math mode or display math mode (see Modes). Both commands are
fragile (see \protect).
In this example each \boldmath command takes place inside an
\mbox,
we have $\mbox{\boldmath \( v \)} = 5\cdot\mbox{\boldmath \( u \)$}$
which means \boldmath is only called in a text mode, here LR
mode, and explains why we must switch LaTeX into math mode to set
v and u.
If you use either command inside math mode, as with Trouble: \(
\boldmath x \), then you get something like ‘LaTeX Font Warning:
Command \boldmath invalid in math mode’ and ‘LaTeX Font Warning:
Command \mathversion invalid in math mode’.