The notation $\operatorname{argmax}(f(x))$ usually denotes the value of $x$ that maximizes $f(x)$.
What happens if $f(x)$ has more than one maximum? In this case we have a set of values of $x$ that all achieve the same maximum value of $f(x)$.
Is there a notation, similar to $\operatorname{argmax}(f(x))$, that explicitly denotes this set of maximiser values? That is, I want to emphasize that I am referring to a set, not to a single value.
A lot of times authors will use $\mathrm{Argmax}$ and $\mathrm{argmax}$ to denote the two cases. Even more often, they will just use $\mathrm{argmax}$ and let the reader discern from context whether it is set-valued or not.