  There are three families of bivariate polynomial maps associated with the
rank-$2$ simple complex Lie algebras $A_2, B_2 \cong C_2$ and $G_2$. It is
known that the bivariate polynomial map associated with $A_2$ induces a
permutation of $\mathbf{F}_q^2$ if and only if $\gcd(k,q^s-1)=1$ for $s=1, 2,
3$. In this paper, we give similar criteria for the other two families. As an
application, a counterexample is given to a conjecture posed by Lidl and Wells
about the generalized Schur's problem.
