Is there a “ping-pong lemma proof” that ⟨x↦x+1,x↦x3⟩\langle x \mapsto x+1,x \mapsto x^3 \rangle is a free group of rank 2?

Let f,g:RR be the permutations defined by f:xx+1 and g:xx3, or maybe even have g:xxp, p an odd prime.

In the book, by Pierre de la Harpe, Topics in Geometric Group Theory section 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 fields by Stephen D. Cohen and A.M.W. Glass

  • The group generated by xx+1 and xxp is free. by Samuel White

  • Arithmetic permutations by S.A. Adeleke and A.M.W. Glass


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