We show that the rational subset membership problem in G can be reduced to the submonoid membership problem in G×H where H is virtually Abelian. We use this to show that there is no algorithm reducing submonoid membership to a finite index subgroup uniformly for all virtually nilpotent groups. We also provide evidence towards the existence of a group G with a subgroup $H<G$ of index 2, such that the submonoid membership problem is decidable in H but not in G.
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Doron Shafrir (2024) studied this question.
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