  We develop techniques for computing the AK invariant of a domain with
arbitrary characteristic. We use these techniques to describe for any field $K$
the automorphism group of $K[X,Y,Z]/(X^n Y - Z^2 - h(X)Z)$, where $h(0) \ne 0$
and $n \geq 2$, as well as the isomorphism classes of these algebras.
