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Let $X,Y$ be connected topological manifolds of the same dimension, both with nonempty boundary. Assume $X$ is compact.

Let $f : X \rightarrow Y$ be a continuous map such that $f$ maps $\partial X$ homeomorphically onto $\partial Y$. Then is $f$ necessarily surjective onto $Y$?

My guess is the answer is "yes", but I'm not sure how to prove this in general. I understand how to prove it directly for the case when $X$ and $Y$ are both the closed $n$-ball, but the proof uses the fact that the $n$-ball with an interior point removed deformation retracts to an $(n-1)$-sphere, and I'm not sure how to generalize this to other manifolds.

Arctic Char
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I.A.S. Tambe
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1 Answers1

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Note: a previous version of this answer had the extra assumption that $Y$ is compact. This version deals with the general case where $Y$ is possibly non-compact.


Yes, this is true. The proof is similar in spirit to the proof that a map of non-zero degree between connected closed manifolds of the same dimension is surjective. $\newcommand{\bdry}{\partial} \newcommand{\iso}{\cong} \newcommand{\Z}{\Bbb{Z}} \newcommand{Int}{\operatorname{Int}}$

Let $n$ be the dimension of the manifolds. By assumption, $f$ induces a map of pairs $(X, \bdry X) \to (Y, \bdry Y)$. As a first step, we will prove:

Proposition A: the map $H_n(X, \bdry X) \to H_n(Y, \bdry Y)$ induced by $f$ is non-zero.

Proof: Consider the following portion of the long exact sequences of the pairs with coefficients in $\Z_2$: $$ \require{AMScd} \begin{CD} H_n(X) @>>> H_n(X, \bdry X) @>>> H_{n-1}( \bdry X)\\ {} @VVV @VVV \\ @. H_{n}(Y, \bdry Y) @>>> H_{n-1}(\bdry Y) \end{CD}$$ where the vertical maps are induced by $f$. We have the following observations:

  1. The map $H_{n-1}(\bdry X) \to H_{n-1}(\bdry Y)$ is an isomorphism since $f$ restricts to a homeo from $\bdry X$ to $\bdry Y$.

  2. $H_n(X) = 0$ since $X$ has the homotopy type of a non-compact manifold (see Proposition 3.29 of Hatcher). It follows that the boundary map $H_n(X, \bdry X) \to H_{n-1}(\bdry X)$ is injective.

  3. $H_n(X, \bdry X) \iso \Z_2$, generated by the relative fundamental class $[X, \bdry X]$.

By inspecting the diagram, we deduce that the map $f_*: H_n(X, \bdry X) \to H_n(Y, \bdry Y)$ is non-zero. $\square$


Now, suppose that $f$ is not surjective. Then, there is a point in the interior of $Y$ which is not in the image of $f$. Indeed, the image of $f$ is closed since $X$ is compact, so if the image contained $\Int(Y)$ it would also contain the closure of $\Int(Y)$, which is $Y$.

Pick then a point $y \in \Int(Y)$ which is not in the image of $f$. Then, $f$ factors as follows: $$ (X, \bdry X) \to (Y - \{ y \}, \bdry Y) \to (Y, \bdry Y) $$ and we have a similar factorization of the map induced on homology. We will now conclude using the following general fact

Proposition B: Let $Z$ be a connected $n$-manifold with boundary. If $Z$ is non-compact, then $H_n(Z, \bdry Z) = 0$.

By applying this to $Z = Y - \{ y \}$, we obtain a contradiction, since we established in Proposition A that $f_*: H_n(X, \bdry X) \to H_n(Y, \bdry Y)$ is non-zero.

Note 1: If $Y$ is non-compact, we can already conclude by applying Proposition B to $Z = Y$, so there is no need to consider $Y - \{ y \}$.

Note 2: as observed by Christophe Boilley in the comments, this proof still works if we only assume that $f$ induces an embedding of $\bdry X$ inside $\bdry Y$.


Proof of Proposition B

I would be grateful if someone knows a reference for this, or a simpler proof.

We will use the following lemma, which can be found in the book Topology and Geometry by Bredon.

Lemma (Bredon, Corollary 7.11, p.346): Let $M$ be an $n$-manifold without boundary and $A \subset M$ be a closed, connected subset. If $A$ is non-compact, then $H_n(M, M - A) = 0$.

Proof of Proposition B: Let $U$ be a collar neighborhood of $\bdry Z$. Let $V = U - \bdry Z$. By pushing $\bdry Z$ into the interior of $Z$ along the collar coordinate, we get a map $(Z, \bdry Z) \to (\Int Z, V)$ which induces isomorphisms in relative homology. Let $A = \Int Z - V$, which is a closed subset of $\Int Z$. By choosing a smaller $U$ if necessary, $A$ will be homeomorphic to $Z$. In particular, $A$ is connected and non-compact. Therefore, we may apply the lemma (with $M = \Int Z$) and conclude: $$ H_n(Z, \bdry Z) \iso H_n( \Int Z, V) = H_n(\Int Z, \Int Z - A) = 0. $$ $\square$

Derived Cats
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    @DerivedCats On the induced map $\partial X\to\partial Y$, on only need the injectivity in homology. Therefore we can weaken the assumption of the homeomorphism to a simple embedding. Then you can factor the morphism $(X,\partial X)\to(Y,\partial Y)$ via a compact submanifold $Z\subseteq Y$ containing $f(X)$ and such that $f(\partial X)\subset \partial Z$. – Christophe Boilley Aug 26 '25 at 11:14
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    @ChristopheBoilley Thanks, I'll add this to the answer when I have some more time. It's easy to find the submanifold $Z$ in the smooth category (by taking an exhaustion function), but curiously I don't know how to justify this in the topological category. – Derived Cats Aug 26 '25 at 12:58
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    @DerivedCats It might be overkill, but I think you can use this. – Thorgott Aug 26 '25 at 16:02
  • @Thorgott Thanks for the link. Unfortunately I don't know how transversality works in the topological category... In the end I used a workaround: see the new version of the answer. – Derived Cats Aug 27 '25 at 03:21
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    @DerivedCats Ah, this is much better anyway. 1. I believe you can slightly (but only slightly) streamline the proof of Proposition B by noting $H_n(Z,\partial Z)\cong H_n(D(Z),Z)$, where $D(Z)$ is the double of $Z$, and then apply Bredon's 7.11 directly. 2. Re: Note 2. I don't think an arbitrary embedding works cause that need not be injective in top homology. – Thorgott Aug 27 '25 at 09:25
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    @Thorgott 1. Ah, the good old double trick. Thanks for the suggestion! 2. In that remark I am still assuming that the manifolds have the same dimension. Hence the image of the embedding is a union of components of $\partial Y$, so the map on homology is indeed injective. – Derived Cats Aug 27 '25 at 13:09