= Invariant vs covariant
Some sources distinguish "invariant" from "covariant" such that under some transformation (typically <Lie group>):
* invariant: the value of $f(x)$ does not change if we transform $x$
* covariant: the form of the equation $f(x)$ does not change if we transform $x$.
TODO examples.
Bibliography:
* https://physics.stackexchange.com/questions/7700/definitions-and-usage-of-covariant-form-invariant-invariant
* https://physics.stackexchange.com/questions/270296/what-is-the-difference-between-lorentz-invariant-and-lorentz-covariant