Radical function in two (or more) variables

Question

I'm reading a paper where the author uses the word radical function for a function $f:\mathbb{R}^{n}\rightarrow \mathbb{R}$. I understand the definition of a radical function if $n=1$, but what if $n>1$?

Answer 1

I would imagine it would be what you think it is in $\mathbb{R}^1$ but with more variables. Instead of $f(x)=\sqrt{x}$, we might have $f(x,y,z)=\sqrt{x^2+y^2+z}$