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.
Figure 1: A pinched torus
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.[1][2]
Definition
A topological space X endowed with a triangulationK is an n-dimensional pseudomanifold if the following conditions hold:[3]
Every (n–1)-simplex 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 σ = σ0, σ1, ..., σk = σ' such that the intersectionσi ∩ σi+1 is an (n−1)-simplex for all i = 0, ..., k−1.
Condition 3 means that X is a strongly connected simplicial complex.[4]
If we require Condition 2 to hold only for (n−1)-simplexes in sequences of n-simplexes 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 n-simplexes satisfying Condition 2.[5]
Decomposition
Strongly connected n-complexes can always be assembled from n-simplexes gluing just two of them at (n−1)-simplexes. However, in general, construction by gluing can lead to non-pseudomanifoldness (see Figure 2).
Figure 2: Gluing a manifold along manifold edges (in green) may create non-pseudomanifold edges (in red). A decomposition is possible cutting (in blue) at a singular edge
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).
Figure 3: The non pseudomanifold surface on the left can be decomposed into an orientable manifold (central) or into a non-orientable one (on the right).
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).
Figure 4: Two 3-pseudomanifolds with singularities (in red) that cannot be broken into manifold parts only by cutting at singularities.
For n≥3, there are n-complexes that cannot be decomposed, even into pseudomanifold parts, only by cutting at singularities.[5]
Related definitions
A pseudomanifold is called normal if the link of each simplex with codimension ≥ 2 is a pseudomanifold.
Examples
A pinched torus (see Figure 1) is an example of an orientable, compact 2-dimensional pseudomanifold.[3]
(Note that a pinched torus is not a normal pseudomanifold, since the link of a vertex is not connected.)
Complex algebraic varieties (even with singularities) are examples of pseudomanifolds.[4]
(Note that real algebraic varieties aren't always pseudomanifolds, since their singularities can be of codimension 1, take xy=0 for example.)
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.[6]
Combinatorial n-complexes defined by gluing two n-simplexes at a (n-1)-face are not always n-pseudomanifolds. Gluing can induce non-pseudomanifoldness.[5]
^Spanier, H. (1966), Algebraic Topology, McGraw-Hill Education, ISBN0-07-059883-5
^ abBrasselet, J. P. (1996). "Intersection of Algebraic Cycles". Journal of Mathematical Sciences. 82 (5). Springer New York: 3625–3632. doi:10.1007/bf02362566. S2CID122992009.
