  Given $f$ in $GL(d,Z)$, it is decidable whether its mapping torus (the
semi-direct product of $Z^d$ with $Z$) may be generated by two elements or not;
if so, one can classify generating pairs up to Nielsen equivalence. If $f$ has
infinite order, the mapping torus of $f^n$ cannot be generated by two elements
for $n$ large enough; equivalently, $f^n$ is not conjugate to a companion
matrix in $GL(d,Z)$ if $n$ is large.
