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In what follows $M$ will be a manifold (without boundary, for simplicity) and $C\subseteq M$ will denote a compact subset. In the paper Deformations of spaces of imbeddings Edwards and Kirby prove the following:

Corollary 1.2: Let $h_t:C\to M$ be an isotopy such that it has an extension to a neighbourhood $U$ of $C$. Then there exists an isotopy $H_t:M\to M$ such that $H_0=1_M$ and $h_t=H_t\circ h_0$.

I would be interested in applying this Corollary to the following situation: $M$ is a compact complex manifold and $C=C_1\cup\dots \cup C_n$ is a normal crossing divisor. For example one can take $C=C_1\cup C_2$ to be the union of two closed submanifolds of (complex) codimension 1 that intersect transversally.

Question: Does any one know if I can apply the Corollary to the situation I am interested in?

My attempt: In this paper https://arxiv.org/pdf/1706.09539.pdf the authors prove that the inclusion $C\to M$ is a cofibration (Theorem 1.1) (they work in an equivariant setting, so just pretend the the group in question is the trivial group). This is not exactly what I was searching for, because it ensures only that I can extend the isotopy to an homotopy.

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    $\begingroup$ Are you asking for topological isotopies or smooth ones? The normal crossing setting is smooth, while Edwards-Kirby setting is topological. Did you check "Stratified Morse theory" by Gorseky-MacPherson? $\endgroup$ Commented Nov 30, 2023 at 14:37
  • $\begingroup$ Sorry, in what sense the normal crossing setting is smooth? A normal crossing divisor is not a manifold (unless there are no crossings) so I think the only definition of isotopy that make sense is the one of Kirby-Edwards...am I wrong? Anyway, thanks for the reference, I will look at it! $\endgroup$ Commented Nov 30, 2023 at 15:06
  • $\begingroup$ The definition of normal crossing that I know involves smoothness: en.wikipedia.org/wiki/…. The paper you linked to contains references to Stratified Morse theory and other works on the subject, so hopefully you can find a reference there. $\endgroup$ Commented Nov 30, 2023 at 15:38
  • $\begingroup$ Thanks @IgorBelegradek, I think I get your point, I will look at the Goresky-MacPherson book! $\endgroup$ Commented Nov 30, 2023 at 15:42
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    $\begingroup$ As long as there is sufficient smoothness and your subset has a regular neighbourhood, the basic proof of isotopy extension works. Perhaps take a look at the proof in Hirsch's Differential Topology textbook. It will give you an idea of ways the application can be generalized. $\endgroup$ Commented Nov 30, 2023 at 16:09

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