Non-Hausdorff manifold

In mathematics, it is a usual axiom of a manifold to be a Hausdorff space, and this is assumed throughout geometry and topology: "manifold" means "(second countable) Hausdorff manifold".

In general topology, this axiom is relaxed, and one studies non-Hausdorff manifolds: spaces locally homeomorphic to Euclidean space, but not necessarily Hausdorff.

Examples

Line with two origins

The most familiar non-Hausdorff manifold is the line with two origins, or bug-eyed line.

This is the quotient space of two copies of the real line

R × {a} and R × {b}

with the equivalence relation

$(x,a)\sim (x,b){\text{ if }}x\neq 0.\;$

This space has a single point for each nonzero real number r and two points 0_a and 0_b. In this space all neighbourhoods of 0_a intersect all neighbourhoods of 0_b, so it is non-Hausdorff.

Further, the line with two origins does not have the homotopy type of a CW-complex, or of any Hausdorff space.^[1]

Branching line

Similar to the line with two origins is the branching line.

This is the quotient space of two copies of the real line

R × {a} and R × {b}

with the equivalence relation

$(x,a)\sim (x,b){\text{ if }}x<0.\;$

This space has a single point for each negative real number r and two points $x_{a},x_{b}$ for every non-negative number: it has a "fork" at zero.

Etale space

The etale space of a sheaf, such as the sheaf of continuous real functions over a manifold, is a manifold that is often non-Hausdorff. (The etale space is Hausdorff if it is a sheaf of functions with some sort of analytic continuation property.)

Notes

1. Gabard, pp. 4–5

References

• Baillif, Mathieu; Gabard, Alexandre, Manifolds: Hausdorffness versus homogeneity, arXiv:math.GN/0609098v1

• Gabard, Alexandre, A separable manifold failing to have the homotopy type of a CW-complex, arXiv:math.GT/0609665v1