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(The question will be interesting for topological/Pl as well but in order to not be too vague I will restrict the meaning of manifold to smooth manifold without boundary).

I'm interested in the function $f: \mathbb{N} \times\mathbb{Z}_{\ge -1} \to \mathbb{N}$ defined as follows:

$$f(n,k) := \inf_{r \in \mathbb{N}}\{\pi_i(Emb(M,\mathbb{R}^r) = 0, \forall i\le k, \forall \text{ $M$ compact connected n-manifold} \}$$

In other words, $f(n,k)$ is the smallest integer $r$ satisfying that for any compact connected $n$-dim manifold $M$ the space $Emb(M,\mathbb{R}^r)$ is $k$-connected.

Examples:

  1. $f(n,-1)$ - This is the smallest integer s.t. any compact connected $n$-manifold can be embedded in $\mathbb{R}^{f(n,-1)}$. The whiteny embedding theorem tells us that $f(n,-1) \le 2n$. This is also the best linear bound since suitable real projective spaces provide counter examples.

  2. $f(n,0)$ - This is the smallest integer s.t. any compact n-manifold has a unique isotopy class of embeddings in $\mathbb{R}^{f(n,0)}$. I think its known (though I don't have a reference) that if $n \ge 2$ then $f(n,0) \le 2n+1$ and I think this is the best possible linear bound here as well (please correct me if this is wrong).

I know this is not a lot of data to go on but it seems reasonable to conjecture that $f(n,k)$ has a linear bound in both $n$ and $k$. Hence:

Question: Is $f(n,k) = O(n,k)$? If so what is the smallest linear bound on $f(n,k)$ in both variables? Is it perhaps $2n+k+1$? (insert optimistic smile).

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  • $\begingroup$ Yes, $2n+k+1$. General position gives $2n+k+2$ and the Whitney trick peels off another dimension. The key is to rewrite $\pi_iEmb(M,\mathbb R^n)$ as a map $M\times S^i\to \mathbb R^n\times S^i$. This is much better than $M\times S^i\to\mathbb R^n$. It is redundant (and you have to make sure that such redundancy is preserved as you manipulate it), but the self-intersections of such maps are the obstruction to parameterized embedding. Extend to a map $M\times D^{i+1}\to \mathbb R^n\times D^{i+1}$. Apply general position to this new map. $\endgroup$ Commented May 29, 2018 at 1:24

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