Date(s) - 11/09/2017
14 h 00 min - 15 h 00 min
Catégories Pas de Catégories
A branched covering of degree d over the sphere determines a finite collection D of partitions of d, called the branch datum. The converse of this assertion is not true. In 1984, A. Edmonds, R. Kulkarni and R. Stong exhibit, for any non-prime integer d, an exceptional collection, i.e. a collection of partitions satisfying the necessary conditions but that is not realizable as a branch datum of any branched covering over S^2.
This realization problem is still open. It was completely solved just for collections involving a “short” partition like :
1. [d], by R. Thom in 1965, via complex polynomials;
2. [d-1,1], by A. Edmonds, R. Kulkarni and R. Stong in 1984, via permutation groups;
3. [d-2,2], by E. Pervova and C. Petronio in 2008, via minimal checker-board graphs.
For other collections are known, at most, partial results.
In this talk, I would like to introduce the problem and some of the techniques used to study it.