# Pseudo-manifold

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$n$-dimensional and closed (or with boundary)

A finite simplicial complex with the following properties:

a) it is non-branching: Each $(n-1)$-dimensional simplex is a face of precisely two (one or two, respectively) $n$-dimensional simplices;

b) it is strongly connected: Any two $n$-dimensional simplices can be joined by a "chain" of $n$-dimensional simplices in which each pair of neighbouring simplices have a common $(n-1)$-dimensional face;

c) it has dimensional homogeneity: Each simplex is a face of some $n$-dimensional simplex.

If a certain triangulation of a topological space is a pseudo-manifold, then any of its triangulations is a pseudo-manifold. Therefore one can talk about the property of a topological space being (or not being) a pseudo-manifold.

Examples of pseudo-manifolds: triangulable, compact connected homology manifolds over $\mathbf{Z}$; complex algebraic varieties (even with singularities); and Thom spaces of vector bundles over triangulable compact manifolds. Intuitively a pseudo-manifold can be considered as a combinatorial realization of the general idea of a manifold with singularities, the latter forming a set of codimension two. The concepts of orientability, orientation and degree of a mapping make sense for pseudo-manifolds and moreover, within the combinatorial approach, pseudo-manifolds form the natural domain of definition for these concepts (especially as, formally, the definition of a pseudo-manifold is simpler than the definition of a combinatorial manifold). Cycles in a manifold can in a certain sense be realized by means of pseudo-manifolds (see Steenrod problem).

How to Cite This Entry:
Pseudo-manifold. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Pseudo-manifold&oldid=42067
This article was adapted from an original article by D.V. Anosov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article