We consider a group-theoretic analogue of the classic subset sum problem. In this brief note, we show that the subset sum problem is NP-complete in the first Grigorchuk group. More generally, we show NP-hardness of that problem in weakly regular branch groups, which implies NP-completeness if the group is, in addition, contracting.
Comment: v3: final version for journal of Groups, Complexity, Cryptology.
arXiv admin note: text overlap with arXiv:1703.07406