Past exam of the mathematics course of the University of Cambridge / 2016 / iii / Paper 104 / 5 / f

Form the HNN extension which centralizes $L$:This is the extension associated to the identity isomorphism $L\to L$; relations on the displayed finite generating set imply $k^{-1}hk=h$ for all $h\in L$. Since $K_{\mathcal{M}}$ is finitely presented and $I,J$ are finite, this is a finite group presentation.

For any $g\in K_{\mathcal{M}}$,The forward implication follows from Britton's lemma: if $g\notin L$, the word $k^{-1}gk{g}^{-1}$ is reduced with two stable letters and cannot be the identity. The reverse implication is the defining centralization relation. Consequently the computable wordsatisfiesAn algorithm for the word problem for a group $G_{\mathcal{M}}$ would therefore decide the nonrecursive set $H_0(\mathcal{M})$, a contradiction. Hence $\boxed{G_{\mathcal{M}}\text{ has unsolvable word problem}}$.