On groups with weak Sierpi\'nski subsets

In a group $G$, a weak Sierpi'nski subset is a subset $E$ such that for some $g,h\in G$ and $a\mathrm{\ne }b\in E$, we have $gE=E\setminus \{a\}$ and $hE=E\setminus \{b\}$. In this setting, we study the subgroup generated by $g$ and $h$, and show that it has a special presentation, namely of the form $G_k=⟨g,h\mid (h^{-1}g{)}^k⟩$ unless it is free over $(g,h)$. In addition, in such groups $G_k$, we characterize all weak Sierpi'nski subsets.