 A free Kleinian group generated by two parabolic transformations

The integer matrices $A=(\begin{array}{cc}1& 2\\ 0& 1\end{array})$ and $B=(\begin{array}{cc}1& 0\\ 2& 1\end{array})$ generate a discrete subgroup. For every nonzero integer $m$, $A^m$ sends $|x|<1$ into $|x|>1$, while $B^m$ sends $|x|>1$, including infinity, into $|x|<1$. The ping-pong lemma identifies their subgroup as the free product of two infinite cyclic groups. This separates the proof of discreteness from the proof of freeness.