## Automorphic sets and braids and singularities.(English)Zbl 0716.20017

Braids, AMS-IMS-SIAM Jt. Summer Res. Conf., Santa Cruz/Calif. 1986, Contemp. Math. 78, 45-115 (1988).
[For the entire collection see Zbl 0651.00010.]
The contents of the survey is as follows. §1 is a brief introduction to some of Artin’s classical results on braid groups, presented in a way fit for future developments. §2 introduces the automorphic sets together with a few categorical definitions obviously related to this notion and gives many examples. §3 deals with the operation of $$B_ n$$ and certain other groups on the cartesian products $$\Delta^ n$$ of automorphic sets $$\Delta$$ and introduces invariants of the orbits, in particular pseudo Coxeter elements, Coxeter diagrams and monodromy groups. This is followed by a discussion of problems related to the operation of $$B_ n$$ on $$\Delta^ n$$ and a report on classical and recent work on these problems. Much of this is related to root systems $$\Delta$$, which are a particularly nice class of automorphic sets. §4 finally explains the applications to singularities.

### MSC:

 20F36 Braid groups; Artin groups 32S30 Deformations of complex singularities; vanishing cycles 14B05 Singularities in algebraic geometry 20B27 Infinite automorphism groups 17B20 Simple, semisimple, reductive (super)algebras 32S40 Monodromy; relations with differential equations and $$D$$-modules (complex-analytic aspects)

Zbl 0651.00010