Let $X$ be a Hausdorff space, and $X' \subset X$. Then $X'$ is collared in $X$ and $X'$ is strongly paracompact in $X'$ if and only if $X'$ is closed in $X$, $X'$ is strongly paracompact in $X$, and $X'$ is locally collared in $X$.
Suppose then that $X$ is a manifold-with-boundary; e.g Hausdorff and locally upper-Euclidean and not necessarily connected. It can be shown that $\partial X$ is closed in $X$, and $\partial X$ is locally collared in $X$. It can also be shown that in a locally compact space, which a manifold-with-boundary is, strong paracompactness is equivalent to paracompactness. Therefore, we have the following theorem:
Let $X$ be a manifold-with-boundary. Then $\partial X$ is collared in $X$ and $\partial X$ is paracompact in $\partial X$ if and only if $\partial X$ is paracompact in $X$.
There are two distinct definitions of a collar, the one used in Brown (Locally flat imbeddings of topological manifolds), and the one used inAn Connelly (A new proof of Brown's collaring theorem). By a collar I refer to Connelly's definition. The definition of Connelly-collar is:
A Connelly-$X$-collar of $X' \subset X$ is a function $h : X' \times [0, 1] \to X$, such that
- $h$ is a closedan embedding,
- $h(x, 0) = x$, for each $x \in X'$,
- $h(X' \times [0, 1))$ is open in $X$
In this case, say thatA subset $X'$$X' \subset X$ is $X$-collared. Since $h$ is closed, we see that $X'$ is closed in $X$.
A Brown-$X$-collar of $X' \subset X$ isif there exists an open embedding $g : X' \times [0, 1) \to X$ such that $g(x, 0) = x$ for each $x \in X'$.
Existence of Connelly$X$-collar implies existence of Brown-collar: given a Connelly-collar $h$, we have that $h|(X' \times [0, 1))$ is a Brown-collar$X'$.
A local Connelly-collarcollar of $X' \subset X$ in $X$ is a pair $(U, h)$, where $U \subset X'$ is open in $X'$, and $h : \overline{U}(X') \times [0, 1] \to X$ is a closedan embedding such that
A subset $X' \subset X$ is locally Connelly-collaredcollared in $X$, if for each $x \in X'$ there exists a local Connelly-collarcollar $(U, h)$ of $X'$ in $X$ such that $x \in U$.
Here are some properties of (Connelly-)collaredcollared subsets, which I think to have proved:
- If $X' \subset X$ is $X$-collared, then $X' \times Y$ is $(X \times Y)$-collared. Applied to manifolds-with-boundary, if $X$ is a manifold-with-boundary, $Y$ is a (boundaryless) manifold, and $\partial X$ is $X$-collared, then $X \times Y$ is a manifold-with-boundary, and $\partial (X \times Y) = \partial X \times Y$ is $(X \times Y)$-collared.
- If $X'_i \subset X_i$ is $X_i$-collared for each $i \in I$, then $\sqcup X'_I$ is $\sqcup X_I$-collared, where $\sqcup$ denotes disjoint sum.
- See here. Let $X$ be Hausdorff, and $X' \subset X$ be locally compact, strongly $X$-paracompact, and closed in $X$. Then $X'$ is Connelly-collared in $X$ if and only if $X'$ is Brown-collared in $X$.
- Applied to a manifold-with-boundary $X$ with $X$-paracompact boundary, $\partial X$ is Connelly-collared if and only if $\partial X$ is Brown-collared.
- The definitions of Connelly-collared and Brown-collared are equivalent for closed sets in locally compact paracompact Hausdorff space, which includes locally compact metric spaces.
- Connelly states (without proof) that the definitions are equivalent when $X$ is a metric space. I don't know whether this is true or not, since I haven't seen a proof anywhere and have not been able to prove or disprove it myself.
- Let $X' \subset X$, $\mathcal{U} \subset \mathcal{P}(X')$ be an $X'$-open cover of $X'$, $V_U = \overline{U}(X') \times [0, 1]$ for each $U \in \mathcal{U}$, and $h : X' \times [0, 1] \to X$. Then $h$ is a collar of $X'$ in $X$ and $\mathcal{U}$ is $X'$-locally finite if and only if $h$ is injective, $(U, h|V_U)$ is a local collar of $X'$ in $X$ for each $U \in \mathcal{U}$, and $\{h(V_U) : U \in \mathcal{U}\}$ is $X$-locally finite.