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This answer suggests that laypeople's intuitive notion of the meaning of these words is consistent with the following claims:

  • A cube has 6 faces, 12 edges, 8 vertices.
  • A cylinder has 3 faces, 2 edges, 0 vertices.
  • A cone has 2 faces, 1 edge, 1 vertex.
  • A sphere has 1 face, 0 edges, 0 vertices.

What formal, mathematical definition best captures this intuition and is consistent with the above claims? For example, the following was suggested in the comments:

I think the definition of the tangent space at a point via equivalence classes of smooth curves works in this situation, and then I believe it's true that the naive count of faces, edges, etc. counts connected components of the subspaces of points whose tangent spaces have the relevant dimensions.

Is this a good approach? If so, does it have an existing name and literature? Are there any other approaches?

Arctic Char
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user76284
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    I don't think there's a need to talk about tangent spaces - it seems simpler to just talk about building the whole shape by gluing together connected manifolds of various dimensions (with the $0$-dimensional ones being vertices, the $1$-dimensional ones being edges, etc.). Basically, a CW-complex. – Noah Schweber Apr 06 '20 at 22:39
  • @NoahSchweber: I'm not convinced that the cylinder is made up of three two-dimensional CW-complexes. =D – user21820 Apr 24 '20 at 10:11

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Partial answer: From generalizations of manifolds:

Orbifolds: An orbifold is a generalization of manifold allowing for certain kinds of "singularities" in the topology. Roughly speaking, it is a space which locally looks like the quotients of some simple space (e.g. Euclidean space) by the actions of various finite groups. The singularities correspond to fixed points of the group actions, and the actions must be compatible in a certain sense.

Stratified space: A "stratified space" is a space that can be divided into pieces ("strata"), with each stratum a manifold, with the strata fitting together in prescribed ways (formally, a filtration by closed subsets). There are various technical definitions, notably a Whitney stratified space (see Whitney conditions) for smooth manifolds and a topologically stratified space for topological manifolds.

CW-complexes: A CW complex is a topological space formed by gluing disks of different dimensionality together. In general the resulting space is singular, and hence not a manifold. However, they are of central interest in algebraic topology, especially in homotopy theory, as they are easy to compute with and singularities are not a concern.

From topologically stratified space:

In topology, a branch of mathematics, a topologically stratified space is a space X that has been decomposed into pieces called strata; these strata are manifolds and are required to fit together in a certain way. Topologically stratified spaces provide a purely topological setting for the study of singularities analogous to the more differential-geometric theory of Whitney.

Basic examples of stratified spaces include manifolds with boundary (top dimension and codimension 1 boundary) and manifolds with corners (top dimension, codimension 1 boundary, codimension 2 corners).

user76284
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