Pseudomanifold Explained

In mathematics, a pseudomanifold is a special type of topological space. It looks like a manifold at most of its points, but it may contain singularities. For example, the cone of solutions of

z2=x2+y2

forms a pseudomanifold.

A pseudomanifold can be regarded as a combinatorial realisation of the general idea of a manifold with singularities. The concepts of orientability, orientation and degree of a mapping make sense for pseudomanifolds and moreover, within the combinatorial approach, pseudomanifolds form the natural domain of definition for these concepts.

Definition

A topological space X endowed with a triangulation K is an n-dimensional pseudomanifold if the following conditions hold:[1]

  1. (pure) is the union of all n-simplices.
  2. Every is a face of exactly one or two n-simplices for n > 1.
  3. For every pair of n-simplices σ and σ' in K, there is a sequence of n-simplices such that the intersection is an for all i = 0, ..., k−1.

Implications of the definition

Decomposition

Strongly connected n-complexes can always be assembled from gluing just two of them at . However, in general, construction by gluing can lead to non-pseudomanifoldness (see Figure 2). Nevertheless it is always possible to decompose a non-pseudomanifold surface into manifold parts cutting only at singular edges and vertices (see Figure 2 in blue). For some surfaces several non-equivalent options are possible (see Figure 3).On the other hand, in higher dimension, for n>2, the situation becomes rather tricky.

Related definitions

Examples

(Note that a pinched torus is not a normal pseudomanifold, since the link of a vertex is not connected.)

(Note that real algebraic varieties aren't always pseudomanifolds, since their singularities can be of codimension 1, take xy=0 for example.)

See also

Notes and References

  1. Brasselet. J. P.. 1996 . Intersection of Algebraic Cycles . Journal of Mathematical Sciences. Springer New York. 82. 5. 3625–3632. 10.1007/bf02362566. 122992009.
  2. PhD . 1904.00306v1. F. Morando. Decomposition and Modeling in the Non-Manifold domain. 139–142.
  3. Baez . John C . Christensen . J Daniel . Halford . Thomas R . Tsang . David C . Spin foam models of Riemannian quantum gravity . Classical and Quantum Gravity . IOP Publishing . 19 . 18 . 2002-08-22 . 0264-9381 . 10.1088/0264-9381/19/18/301 . 4627–4648. gr-qc/0202017 .