$p$ is a large prime number. Consider the following function $F:\mathbb Z^*_p \times \mathbb D\rightarrow\mathbb Z^*_p$ where $\mathbb D=2,....,p-1$.
$F_k(x)=x^k \bmod p$
Proof that it's not a secure pseudorandom function.
What I‘ve tried:
I give a value to $x_1$ and obtain $y_1$, then I set $x_2 =y_1+p$. At the end $y_1$ is equal to $y_2$. Could this be right or is it totally wrong?