# How do we express the union of closed neighborhoods of a cover with pairwise boundary intersection for a manifold $M$?

Consider local charts $(U_{\alpha},\varphi_{\alpha})$ of an $n$-dimensional manifold $M$ with closed neighborhoods $U_{\alpha}$, $\varphi_{\alpha}:U\to V\subset\mathbb{R}^n$, and $\alpha\in A$ an element of the index set $A$. Then we choose an atlas $\mathcal{A}=\{(U_{\alpha},\varphi_{\alpha})|\alpha\in A\}$ in such a way that for any two distinct $i,j\in A$ in the atlas, the intersection of $U_i$ and $U_j$ is their shared boundary (a subset of $\partial U_i$ and $\partial U_j$), i.e. $U_i\cap U_j=\partial U_i\cap\partial U_j\ne\emptyset$ and the union of the closed neighborhoods still covers $M$.

What can the union of the neighborhoods be expressed as, besides the tautological $\bigcup_{\alpha\in A}U_{\alpha}$? That is, we want a covering of $M$ where the weak topology is such that the neighborhoods only intersect on boundaries, which is $\bigcup_{\alpha\in A}\text{Int}(U_{\alpha})\cup{\text{boundary intersections}}$.

Any help would be much appreciated! Thanks in advance.

• I have never seen something like a chart defined on a closed $U_\alpha \subset M$. Even if you can give an interpretation for such a concept, you can only expect $U_i \cap U_j \subset \partial U_i \cap \partial U_j$. – Paul Frost Aug 22 '18 at 18:27
• Yes, it should technically be open, but I don't know how else I would define boundaries. @PaulFrost – Sergio Charles Aug 22 '18 at 18:31
• Then does the question make sense? What is the purpose of these $U_\alpha$? – Paul Frost Aug 22 '18 at 18:35
• I want a cover for a manifold with essentially no overlaps (except on the boundaries of the neighborhoods covering $M$), if that makes sense. @PaulFrost – Sergio Charles Aug 22 '18 at 18:47

You do not mention what kind of manifolds you are interested in (topological, smooth, $C^r$?), but let us ignore that point.
A chart $\varphi : U \to V \subset \mathbb{R}^n$ for $M$ is always defined on an open $U \subset M$. The boundary of $U$ can be understood as the topological boundary of the subspace $U$ of $M$. But then $\varphi$ is not defined on the boundary and you do not get something like "a chart on $\overline{U}$". The only interpretation that could make sense is this: For each closed ball $B \subset V$ consider the pair $(\varphi,B)$. Then $\varphi^B = \varphi \mid_{\varphi^{-1}(B)} : \varphi^{-1}(B) \to B$ may be regarded as a closed chart for $M$. It is essential that it is the restriction of an ordinary chart. Instead of closed balls you may also take closed $n$-simplices, closed $n$-cubes etc.
If you consider topological manifolds, then all these objects $B$ and $\varphi^{-1}(B)$ are manifolds with boundary. In the smooth case this is true for balls $B$, for simplices or cubes we get a manifold with corners (see Definition of "a topological manifold with corners".).
• @Multivariablecalculus So they are smooth manifolds and admit a piecewise-linear triangulation. You may choose this triangulation arbitrarily fine so that each $n$-simplex is contained in the domain of a chart. See for example math.harvard.edu/~lurie/937notes/937Lecture3.pdf. – Paul Frost Aug 23 '18 at 12:27