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
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]
- (pure) is the union of all n-simplices.
- Every is a face of exactly one or two n-simplices for n > 1.
- 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
- Condition 2 means that X is a non-branching simplicial complex.
- Condition 3 means that X is a strongly connected simplicial complex.
- If we require Condition 2 to hold only for in sequences of in Condition 3, we obtain an equivalent definition only for n=2. For n≥3 there are examples of combinatorial non-pseudomanifolds that are strongly connected through sequences of satisfying Condition 2.[2]
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.
- In general, for n≥3, n-pseudomanifolds cannot be decomposed into manifold parts only by cutting at singularities (see Figure 4).
- For n≥3, there are n-complexes that cannot be decomposed, even into pseudomanifold parts, only by cutting at singularities.
Related definitions
- A pseudomanifold is called normal if the link of each simplex with codimension ≥ 2 is a pseudomanifold.
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.)
- Thom spaces of vector bundles over triangulable compact manifolds are examples of pseudomanifolds.
- Triangulable, compact, connected, homology manifolds over Z are examples of pseudomanifolds.
- Complexes obtained gluing two 4-simplices at a common tetrahedron are a proper superset of 4-pseudomanifolds used in spin foam formulation of loop quantum gravity.[3]
- Combinatorial n-complexes defined by gluing two at a are not always n-pseudomanifolds. Gluing can induce non-pseudomanifoldness.
See also
Notes and References
- Brasselet. J. P.. 1996 . Intersection of Algebraic Cycles . Journal of Mathematical Sciences. Springer New York. 82. 5. 3625–3632. 10.1007/bf02362566. 122992009.
- PhD . 1904.00306v1. F. Morando. Decomposition and Modeling in the Non-Manifold domain. 139–142.
- 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 .