Sorry for my bad English.
Let $p$ be a prime.
We say $j\in \mathbb{F}_{p^2}$ is supersingular if the corresponding elliptic curve to $j$ is supersingular.
So there is polynomial $f(X)\in \mathbb{F}_{p^2}[X]$ s.t. $f(j)=0$ iff $j$ is supersingular.
I have two questions.
First, $f(X)\in \mathbb{F}_p$?
Second, does $f(X)$ have another equivalent definition?