Brouwer degree, domination of manifolds, and groups presentable by products
For oriented connected closed manifolds of the same dimension, there is a transitive relation: M dominates N, or M >= N, if there exists a continuous map of non-zero degree from M onto N. Section 1 is a reminder on the notion of degree (Brouwer, Hopf), Section 2 shows examples of domination and a first set of obstructions to domination due to Hopf, and Section 3 describes obstructions in terms of Gromov's simplicial volume. In Section 4 we address the particular question of when a given manifold can (or cannot) be dominated by a product. These considerations suggest a notion for groups (fundamental groups), due to D. Kotschick and C. Löh: a group is presentable by a product if it contains two infinite commuting subgroups which generate a subgroup of finite index. The last section shows a small sample of groups which are not presentable by products; examples include appropriate Coxeter groups.
Brouwer degree, domination of manifolds, simplicial volume
Primary: 55M25, 57N65
|9th July 2015
|16th March 2016
|29th January 2019