We defined semi-Thue processes and showed (Post, 1947) that for each e there is a semi-Thue process such that the word problem for is Turing-above the halting problem for .

We then proceeded to define Thue processes and showed that they give rise to semigroups. It follows (Post; Markov) that there is a presentation of a semigroup with an unsolvable word problem.

It is easy to modify a Thue process so the semigroup it generates is indeed a group. A modification of the arguments for semigroups shows (Novikov; Boone) that the word problem for groups is unsolvable. In fact (Adjan,Rabin) if is a class of groups closed under isomorphisms and subgroups which neither contains nor is disjoint from the class of finitely presented groups, then there is no algorithm that determines from a given group presentation whether it is in .

Additional references:

Modular machines, the word problem for finitely presented groups, Collins’ theorem, by Aanderaa, Cohen. In Word problems II. The Oxford book, Adjan, Boone, Higman, eds. North-Holland (1980), 1-16.

Modular machines and the Higman-Clapham-Valiev embedding theorem, by Aanderaa, Cohen. In Word problems II. The Oxford book, Adjan, Boone, Higman, eds. North-Holland (1980), 17-28.

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