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Let $M\subset\mathbb{R}^n$ be a smooth submanifold of dimension $m<n$, and a tubular neighborhood $\mathcal{V}=E(\{(x,v)\in NM:|v|<\delta(x)\})$ (diffeomorphic image of the smooth map $E:NM\to\mathbb{R}^n$ defined by $E(x,v)=x+v$) with $\delta:M\to]0,\infty[$ continuous function.

Fixed a point $p$, we can consider a local parametrization $\varphi:\mathbb{R}^m\to U_p\subset M$ with $\varphi(0)=p$ such that $T_pM=\varphi'(0)(\mathbb{R}^m)$.

From this, it can be shown that there exists a nbd $B_r(0)\subset\mathbb{R}^m$ such that the part $p+\varphi'(0)(B_r(0))$ of the affine tangent space $p+T_pM$ is contained in the tubular nbd $\mathcal{V}$, and there exists other nbd $V_p\subset U_p$ such that the affine normal spaces $x+N_xM$ intersects $p+\varphi'(0)(B_r(0))$ in a unique point $p_x$ for all $x\in V_p$ (show the image below in the case $n=3$ and $m=1$).

enter image description here

My question:

Taking into account that the normal bundle $NM$ is a smooth submanifold of $\mathbb{R}^{2n}$, it can be proven that the map $x\mapsto p_x$ (well defined in $V_p$) is smooth too?

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  • $\begingroup$ Your picture really is in $\Bbb R^2$. You should be drawing $V$ as a solid tube (with circular cross-sections) if you're in $\Bbb R^3$. Also, having $V$ already, writing $V_p$ for a subset of $U_p$ is beyond confusing. When $p=x$ you're intersecting orthogonal affine subspaces; for $x$ near $p$, they are almost orthogonal and hence have a unique intersection. Can you set this up as an application of the implicit function theorem to prove smoothness? $\endgroup$ Aug 21, 2022 at 20:37
  • $\begingroup$ Thanks for the hints, I was able to solve it locally in two ways, one was using the implicit function theorem and other was using transversality theory. I will respond twice to the post with images, showing the two ways. I would appreciate if you can check them. $\endgroup$
    – Ludwik
    Aug 22, 2022 at 16:18
  • $\begingroup$ I find your written solutions difficult to read. Something seems slightly fishy with the second, but it might be more helpful to write it without component-by-component notation. Your approach is, however, correct. The first seems right, but I wouldn't call this transversality theory. Knowing that the preimage of a regular value is a submanifold is elementary and doesn't even begin to use Sard's Theorem. $\endgroup$ Aug 22, 2022 at 21:14
  • $\begingroup$ Thanks Ted, I'll have it in mind. $\endgroup$
    – Ludwik
    Aug 24, 2022 at 0:02

2 Answers 2

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Locally solved using transversality theory:

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Locally solved using the implicit function theorem:

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