Past exam of the mathematics course of the University of Cambridge / 2024 / iii / Paper 133 / 1 / b / Solution

Introduce $b=t^{-1}at$. The presentation may be written as the HNN extensionof the Baumslag-Solitar group $\mathrm{BS}(1,2)=⟨a,b\mid ba{b}^{-1}=a^2⟩$, with stable letter $t$ identifying the infinite cyclic subgroups $⟨a⟩$ and $⟨b⟩$. Britton's lemma embeds the base group in the HNN extension. In particular, $a$ has infinite order, so $G$ is infinite. It is also nonabelian because $ba{b}^{-1}=a^2\ne a$.