On the subgroup membership problem in bounded automata groups

Abstract: We are interested in the generalised word problem (aka subgroup membership problem) for stabiliser subgroups of groups acting on rooted $d$-regular trees. Stabilisers of infinite rays in the tree are not finitely generated in general, and so the problem is not even well posed unless the infinite ray has a finite description, for example, if the ray is eventually periodic as an infinite word over the alphabet with $d$ letters. We show that for bounded automata groups, the membership problem for such subgroups is solvable by proving that it forms an ET0L language that is constructable. Exploiting this, we give a recursive formula for the associated generating function. We also show that, in general, the membership problem for the stabiliser of an infinite ray in a bounded automata group cannot be described using context-free languages.