Ping-pong for basis-conjugating HNN-extension of free group

Abstract: We construct a normal form for a multiple HNN-extension of a free group by basis-conjugating embeddings. We provide sufficient conditions on a collection of subgroups to fulfill the requirements of ping-pong lemma. Recall that the pure braid group splits as a semidirect product of free groups. Using our result, we show that certain braids from the first two summands $F_n \rtimes F_{n-1} \subset P_{n+1}$ generate a free subgroup.