Path-lifting property for projections onto open sets of $\mathbb{R}^n$

89 Views Asked by At

The question is about path-lifting property for projections that possibly are not coverings.

Let $E, F$ be finite dimensional normed spaces, $A\subset E$ be a connected open set, $f:A\times F\to F$ be a continuously differentiable mapping. It is assumed that the set $\Gamma=f^{-1}(0)$ is non-empty and for each $(x,y)\in\Gamma$ the derivative $\partial_y f(x,y)$ is invertible; finally, $G$ is a connected component of $\Gamma$ such that $pr_1(G)=A$ ($pr_1$ being a local homeomorphism of $G$ onto $A$).

The problem is to show that any continuous mapping $\gamma:[0,1]\to A$ can be lifted to a continuous mapping $\tilde{\gamma}:[0,1]\to G$ (i.e. $pr_1\circ\tilde{\gamma}=\gamma$).

I know how to prove it under some additional assumptions. First is that $pr_2(G)$ is bounded, second (more general) is that $pr_1:G\to A$ is a covering. Without these I am not sure that the result is true, but on the other hand I can't find a counterexample with $A$ being an open set of $E$ (I am aware about examples of mappings onto $S^1$ without path-lifting property that were discussed here, but is it possible to adapt them to my case?)

Thanks for the help!

1

There are 1 best solutions below

0
On

It turned out that the assertion in the question is false in general. An example follows.

Let $E=F=\mathbb{C},$ $A=\mathbb{C}\setminus\{0\},$ and $f(x,y)=x-(e^y-1)^2.$ Then the set of zeroes of $f$ is $$ \Gamma=\{((e^y-1)^2,y):y\in\mathbb{C}\setminus 2\pi i \mathbb{Z}\} $$ (values $y\in 2\pi i \mathbb{Z}$ are excluded because we must have $x\ne 0$). This set is connected as a continuous image of the connected set $\mathbb{C}\setminus 2\pi i \mathbb{Z}.$ Next, for every $(x,y)\in\Gamma$ the derivative $$ D_yf(x,y)=-2e^y(e^y-1)\ne 0 $$ (again becase $x\ne 0$).

Finally, $pr_1(\Gamma)=A.$ Indeed, every nonzero $x$ can be written in the form $x=e^{\log|x|+i\xi}.$ If $|x|\ne 1$ then $1+e^{\frac{1}{2}\log |x|+i\frac{\xi}{2}}\ne 0$ and the equation $e^y=1+e^{\frac{1}{2}\log |x|+i\frac{\xi}{2}}$ has solutions in $y.$ If $|x|=1$ then $x=e^{i\xi}$ for some $\xi\not\in 2\pi(2\mathbb{Z}+1)$ and again $$ 1+e^{\frac{1}{2}\log |x|+i\frac{\xi}{2}}=1+e^{i\frac{\xi}{2}}\ne 0 $$ So, $A,f$ and $\Gamma$ verify all assumptions (in particular, $pr_1$ is a local homeomorphism of $\Gamma$ onto $A$). However, the path $\gamma(t)=e^{it},$ $0\leq t\leq 2\pi,$ in $A$ can't be lifted to $\Gamma.$ Indeed, assume that there exists a path $\psi:[0,2\pi]\to\mathbb{C}$ such that $$ (e^{\psi(t)}-1)^2=e^{it}, \ 0\leq t\leq 2\pi. $$ Then for each $t$ $$ e^{\psi(t)}=1\pm e^{i\frac{t}{2}} $$ By continuity, there must be either $+1$ for all $t,$ or $-1$ for all $t.$ In the first case the equality $$ e^{\psi(t)}=1+ e^{i\frac{t}{2}} $$ fails for $t=2\pi$. In the second case the equality $$ e^{\psi(t)}=1- e^{i\frac{t}{2}} $$ fails for $t=0.$