Let f,g:R→R be the permutations defined by f:x↦x+1 and g:x↦x3, or maybe even have g:x↦xp, p an odd prime.

In the book, by Pierre de la Harpe,

Topics in Geometric Group Theorysection II.B.40, as a research problem, it asks to find an appropriate “ping-pong” action to show that the group, under function composition, G=⟨f,g⟩ is a free group of rank two.Is there such a proof? That is, is there a proof where the key insight is having that group act in such a way to apply the ping-pong lemma(table-tennis lemma)? I have not been able to find such a proof either by working on it, or in the literature.

Maybe we don’t have such a proof but do we have a proof that G contains a free subgroup of rank two, akin to proofs for torsion-free hyperbolic group, or the Tits alternative. I am not sure how obvious it is that G is hyperbolic, or linear. I am guessing it is not obvious that it is linear since I would suspect a ping-pong proof would come out of that pretty quickly.

Note that there are proofs of this theorem, but as far as I know, they do not use the ping-pong lemma.

The only proofs of the result(and more general things) I know of are in :

Free groups from fieldsby Stephen D. Cohen and A.M.W. Glass

The group generated by x↦x+1 and x↦xp is free.by Samuel White

Arithmetic permutationsby S.A. Adeleke and A.M.W. Glass

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