Past exam of the mathematics course of the University of Cambridge / 2016 / iii / Paper 104 / 6 / b

An isomorphism-invariant Markov property of finitely presented groups $\rho$ has two finitely presented witnesses: a group $A_{+}$ with $\rho$, and a group $A_{-}$ which cannot embed in any finitely presented group having $\rho$. In symbols,The second condition is an obstruction to embedding, stronger than merely saying that $A_{-}$ itself fails the property.

Let $U$ be a fixed finitely presented group with unsolvable word problem for a group. Form the finitely presented free product $P=U\ast A_{-}$. Given a word $w$ in the generators of $U$, apply part (a) to this $P$ and $w$, and output the finite presentation ofIf $w=1$ in $U$, it is also $1$ in $P$, so $P(w)$ is trivial and $B(w)\cong A_{+}$ has $\rho$. If $w\ne 1$ in $U$, its image stays nontrivial in the free product $P$, and $P$ embeds in $P(w)$. Therefore $A_{-}$ embeds in $B(w)$, which cannot have $\rho$. We have the effective equivalenceAn algorithm recognizing whether an arbitrary finite group presentation has $\rho$ would decide the unsolvable word problem for a group $U$. This contradiction proves the Adian–Rabin theorem: no Markov property of finitely presented groups is algorithmically decidable.