Published online by Cambridge University Press: 20 November 2018
An epimorphism   $\phi :\,G\,\to \,H$  of groups, where
 $\phi :\,G\,\to \,H$  of groups, where   $G$  has rank
 $G$  has rank   $n$ , is called coessential if every (ordered) generating
 $n$ , is called coessential if every (ordered) generating   $n$ -tuple of
 $n$ -tuple of   $H$  can be lifted along
 $H$  can be lifted along   $\phi $  to a generating
 $\phi $  to a generating   $n$ -tuple for
 $n$ -tuple for   $G$ . We discuss this property in the context of the category of groups, and establish a criterion for such a group
 $G$ . We discuss this property in the context of the category of groups, and establish a criterion for such a group   $G$  to have the property that its abelianization epimorphism
 $G$  to have the property that its abelianization epimorphism   $G\,\to \,{G}/{[G,G]}\;$ , where
 $G\,\to \,{G}/{[G,G]}\;$ , where   $[G,\,G]$  is the commutator subgroup, is coessential. We give an example of a family of 2-generator groups whose abelianization epimorphism is not coessential. This family also provides counterexamples to the generalized Andrews–Curtis conjecture.
 $[G,\,G]$  is the commutator subgroup, is coessential. We give an example of a family of 2-generator groups whose abelianization epimorphism is not coessential. This family also provides counterexamples to the generalized Andrews–Curtis conjecture.