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The following problem that was posed by Grothendieck: Let u W H ! G be a homomorphism of nitely presented residually nite groups for which the extension O u W y H ! y G is an isomorphism. Is u an isomorphism? The problem was solved in the negative by Bridson and Grunewald who produced many examples of groups G and proper subgroups u W H ,! G for which O u(More)
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