This is a survey of results of the profinite approach to decidability problems in the theory of finite semigroups.

In this paper we determine the automorphism groups of profinite braid with four or more strings in terms Grothendieck-Teichm\"uller group.

We generalize the model theory of small profinite structures developed by Newelski to the case of compact metric spaces considered together with compact groups of homeomorphisms.

We compute the completion of the groups SLn(Zt]) and SLn (Zt;t ?1 ]) relative to the obvious homomorphisms to SLn (Q); this is a generalization of the classical Malcev completion. We also make partial computations of the rational second cohomology of these groups.

We generalize the notion of a projective profinite group to a projective pair of a profinite group and a closed subgroup. We establish the connection with Pseudo Algebraically Closed (PAC) extensions of PAC fields: Let M be an algebraic extension of a PAC field K. Then M/K is PAC if and only if the corresponding pair of absolute Galois groups (Gal(M),Gal(K)) is projective. Moreover any projecti...

We associate the iterated block product of a bimachine with a deterministic Turing machine. This allows us to introduce new algebraic notions to study the behavior of the Turing machine. Namely, we introduce double semidirect products through matrix multiplication of upper triangular matrices with coefficients in certain semigroups, which leads in turn to the study of the iterations of bimachin...