nLab
branched manifold

Idea

Smooth branched $n$-manifolds are a generalization of smooth manifolds which may have ill-defined tangent spaces at certain “branch loci”, where however they are required to have a well defined tangent $n$-plane.

Branched $n$-manifolds arise for instance as quotients of foliations.

Definition

Let $X$ be a metrizable space with

1) a collection $\{U_i\}$ of closed subsets of $X$

2) for each $U_i$ a finite collection $\{D_{ij}\}$ of closed subsets of $U_i$

3). for each $i$ a map $p_i:U_i\to D_i^n$ to a closed $n$-disk of class $C^k$ in ${ℝ}^n$

such that

a) ${\cup }_i{D}_{ij}=U_i$ and ${\cup }_i{U}_i^{\circ }=X$

b) $p_i{\mid }_{D_{ij}}$ is a homeomorphism onto its image which is a closed $C^k$ n-disk relative to $\partial D_i^n$.

c) There is a “cocycle” of diffeomorphisms $\{{\alpha }_{i^{\prime }i}\}$ of class $C^k$ such that $p_{i^{\prime }}={\alpha }_{i^{\prime },i}\circ p_i$ when defined. The domain of ${\alpha }_{i^{\prime }i}$ is $p_i(U_i\cap U_{i^{\prime }})$.

Then $X$ is called branched n-manifold of class $C^k$.

This appears as Williams, def. 1.0 ns.

If $X$ satisfies this set of axioms with b) replaced by

$b_{\mathrm{ns}})$ $p_i{\mid }_{D_{ij}}$ is a homeomorphisms onto $D_i^n$

$X$ is called nonsingular branched $n$-manifold of class $C^k$.

Related concepts

• orbifold

References

• Robert F. Williams, Expanding attractors, Publications mathématique d l’IHÉS, tome 43 (1974) (numdam)

• Dusa McDuff, Groupoids, branched manifolds and multisections, J. Symplectic Geom. Volume 4, Number 3 (2006), 259-315 (project euclid)

Discussion relating to orbifolds is in

• Dusa McDuff, Groupoids, branched manifolds and multisections (arXiv:math/0509664)