Branched surface

In mathematics, a branched surface is a generalization of both surfaces and train tracks.

## Definition

[edit]

A surface is a space that locally looks like  R 2 {\displaystyle \mathbb {R} ^{2}} (a Euclidean space, up to homeomorphism).

Consider, however, the space obtained by taking the quotient of two copies A,B of  R 2 {\displaystyle \mathbb {R} ^{2}} under the identification of a closed half-space of each with a closed half-space of the other. This will be a surface except along a single line. Now, pick another copy C of  R {\displaystyle \mathbb {R} } and glue it and A together along halfspaces so that the singular line of this gluing is transverse in A to the previous singular line.

Call this complicated space K. A branched surface is a space that is locally modeled on K.[1]

## Weight

[edit]

A branched manifold can have a weight assigned to various of its subspaces; if this is done, the space is often called a weighted branched manifold.[2] Weights are non-negative real numbers and are assigned to subspaces N that satisfy the following:

* N is open.
* N does not include any points whose only neighborhoods are the quotient space described above.
* N is maximal with respect to the above two conditions.

That is, N is a component of the branched surface minus its branching set. Weights are assigned so that if a component branches into two other components, then the sum of the weights of the two unidentified halfplanes of that neighborhood is the weight of the identified halfplane.

## See also

[edit]

* Branched covering
* Branched manifold

## References

[edit]

1. ^ Li, Tao. "Laminar Branched Surfaces in 3-manifolds." Geometry and Topology 6.153 (2002): 194.
2. ^ Shields, Sandra. "The stability of foliations of orientable 3-manifolds covered by a product." Transactions of the American Mathematical Society 348.11 (1996): 4653-4671.