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Suppose $M$ is a smooth manifold (compact if desired) with a cell structure or other nice stratification.

Call a path $\gamma : [0,1] \to M$ transverse to the stratification if there is a finite sequence $0 < t_1 < \cdots < t_k < 1$ such that for each $t\ne t_i$, $\gamma(t)$ is in the interior of a top dimensional cell, and for each $i$, $\gamma(t_i)$ is in the interior of a codimension-1 cell.

I’m not sure what is the correct notion of transverse for a path homotopy, but it should be something like this: using $s$ as the homotopy parameter, $\gamma_s(t)$ is in the interior of cells of codimension $c\leq 2$ for all $s$ and $t$, of codimension $c\leq 1$ for all but finitely-many pairs $(s,t)$, and of codimension $0$ for all but finitely-many $t$ for each $s$.

(So at most finitely-many of the paths $\gamma_s$ are transverse paths, and the exceptional $\gamma_s$'s only pass through codimension-$2$ cell interiors, and only finitely-many times.)

I guess this requirement could be made nicer since I'm not excluding things like a path touching a wall without passing through it.

Anyway, when are the following true?

  1. Every path $\gamma : [0,1] \to M$ is homotopic to a path transverse to the stratification.

  2. Every homotopy between transverse paths is itself homotopic to a transverse homotopy.

Ideally the first condition leads to generators or an enumeration of $\pi_1(M)$ as cycles along the dual graph of the stratification, and the second condition gives relations.

But, I'm not sure what further niceness assumptions are needed regarding, for example, how many top-dimensional cells can meet along a given codimension-1 or 2 cell.

Examples: $\overline{M_{0,n}}(\mathbb{R})$ and the cactus group; $\mathrm{Conf}_n(\mathbb{R}^2)$ and the braid group.

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  • $\begingroup$ For 1 I would be very surprised if it is not possible to perturb a path away from the $(n-2)$-skeleton (involving path lifting to the boundary map of a higher dimensional disk and then perturbing the lift), Then it seems we can arrange the condition by some local argument (involving path lifting to the boundary of the disk and then perturbing the lift). I am not 100% sure but can't really see how it would go wrong. $\endgroup$
    – Nick L
    Commented Feb 7, 2022 at 13:01

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