Jordan Curve (high level, graduation level)

Hi all

I have an exercice which is at a rather high level. I post it here, because I dont know the general level of the forum. But let see if I have some answers.

Let $\displaystyle \Gamma$ be a Jordan closed curve. Set $\displaystyle \epsilon>0 $

and set:

$\displaystyle \Gamma_{\epsilon}=\left\{z\in\mathbb{C};d(z,\Gamma )\leq\epsilon\right\}$

1) Let $\displaystyle \gamma:[a,b]\rightarrow\mathbb{C}$ a regular parametrization of $\displaystyle \Gamma$ which is $\displaystyle C^2$.

For $\displaystyle \epsilon>0$, we note $\displaystyle R_{\epsilon}$ the rectangle $\displaystyle [a,b]\times[-\epsilon,\epsilon]$ and we set $\displaystyle \Phi_{\epsilon}\rightarrow\mathbb{C}$ by :

$\displaystyle \Phi_{\epsilon}(t,s)=\gamma(t)+sN(t)$

where $\displaystyle N(t)$ is the normal vector to $\displaystyle \gamma$ at the point $\displaystyle t$.

I have shown that if $\displaystyle \epsilon$ is small enough, then $\displaystyle \Phi_\epsilon$ is a $\displaystyle C^1 $ diffeomorphism and that $\displaystyle Im(\Phi_\epsilon)=\Gamma_\epsilon$

Now, I have to show that for \epsilon small enough,

we have:

$\displaystyle m(\Gamma_\epsilon)=\int^b_a\left(\int^{\epsilon}_{-\epsilon}

\left|\gamma'(t)\right|+s\varphi(t)ds\right)dt$

I don't know how to do this. I know that I have to use the jacobien, the integral transformation (Multiple integral - Wikipedia, the free encyclopedia)

Thank you