I currently study special relativity and some authors write stuff like: $$ r^\mu = \left(ct, \vec x\right) $$
This is awful since $\mathsf r$ is a vector, and $r^\mu$ ist just a single component of that vector.
Now I am wondering whether something I write occasionally is similarly wrong: As far as I know, I can define a function $f$ like $f\colon x \mapsto x^2$ or $f(x) = x^2$. Sometimes I just write $f = x^2$. Would that be incorrect in a strict sense?