Consider the following interesting theorem (7.5.7, p.294 in Topology and Groupoids by Ronald Brown):

Gluing theorem for adjunction spaces: Suppose that we have the following commutative diagram of topological spaces and continuous maps:

    alt text

where $\varphi_{A}$, $\varphi_{X}$ and $\varphi_{Y}$ are homotopy equivalences, and the inclusions $i$ and $i'$ are closed cofibrations. Then the map

$$ \varphi:X\cup_{f}Y\rightarrow X'\cup_{f\phantom{l}'}Y' $$

induced by $\varphi_{A}$, $\varphi_{X}$ and $\varphi_{Y}$ is a homotopy equivalence.

In this post I asked a question that would become a corollary if the gluing theorem were true for arbitrary cofibrations $i$ and $i'$. I would like to know if this is the case. Otherwise, does anybody know of a counterexample?

Perhaps, it is possible that although the map $\varphi$ may not in general be a homotopy equivalence when the cofibrations $i$ and $i'$ are arbitrary, the adjunction spaces $Y\cup_{f}X$ and $Y'\cup_{f'}X'$ will nonetheless be homotopy equivalent. A possible approach to this would be to try to find a third space containing both as deformation retracts.

Note on notation: That an inclusion map $j:B\rightarrow Z$ is a closed cofibration means that it is a cofibration (i.e., the pair $(B,Z)$ has the homotopy extension property) and the set $B$ is closed in $X'$.

Thank you

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    $\begingroup$ The result you seek is given as propositions 5.3.2 and 5.3.3 of tom Dieck's book "Algebraic topology". $\endgroup$ May 6 '12 at 6:09

Yes, this is true even for not necessarily closed cofibrations. If you want a single source that gives a complete proof, then the only one that comes to my mind is this preprint.

Definition 1.1.1 introduces cofibration categories and then Lemma 1.4.1 says that the desired result holds in any cofibration category. Section 3.1 contains a detailed proof that the category of topological spaces equipped with Hurewicz cofibrations and homotopy equivalences is a cofibration category and thus the lemma applies. The crux of the matter is that acyclic cofibrations are closed under pushouts and this follows from a classical result of Dold (Lemma 3.1.9) that acyclic cofibrations admit deformation retractions, which doesn't depend on closedness.

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    $\begingroup$ It is good to see these generalisations of the gluing lemma for homotopy equivalences. I can't resist pointing out that this lemma first appeared in Brown, R. Elements of Modern Topology. McGraw-Hill Book Co., New York (1968). $\endgroup$ May 6 '12 at 9:51
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    $\begingroup$ @Ronnie: Unfortunately the works cited above do not seem to mention this fact, or give any other reference regrading the gluing lemma for that matter. $\endgroup$
    – Victor
    May 6 '12 at 11:32
  • $\begingroup$ I add that an advantage of the proof given in "Topology and Groupoids" is that it gives control over the homotopies involved. The result itself evolved from generalising the standard fact that a homotopy equivalence $Y \to Z$ of spaces induces an isomorphism of homotopy groups: now replace the pair $(S^n,a)$ by a pair $(X,A)$ and go through the same argument, to get a useful result on maps of pairs. $\endgroup$ Oct 8 '12 at 15:51
  • $\begingroup$ It would be interesting to have an example where the proposed generalisation to non closed cofibrations was crucial. $\endgroup$ May 11 '14 at 15:58
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    $\begingroup$ I think the general result in a cofibration category is of interest but I would also like to know of significant topological examples where the closed hypothesis is dropped. If anyone wants to see the original proof it is available in this upload of Chapter 7 of Topology and Groupoids: pages.bangor.ac.ul/~mas010/pdffiles/TandGCh7.pdf $\endgroup$ Oct 13 '14 at 20:59

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