Quite recently Eliyahu Rips and Arye Juhasz constructed an Engel but not locally nilpotent group, i.e. group which satisfies for some positive $n$ the identity $underbrace{[x,y],y,dots,y]dots]}_n=e$.This group has non-postitive curvature and big commutative parts, some parts have small cancellation and some commute. – This group looks in some sense like a ring, and group multiplication behaves sometimes like multiplication and sometimes like addition. The theory of canonic forms of this group is applicable for rings, in particulary in skew field construction. In different sense some semigroup constructions can be transformed to rings. There is a hope nowdays to develop a geometric ring theory.
- Séminaire irrégulier