  In this work we consider piecewise smooth vector fields $X$ defined in
$\R^n\setminus \Sigma$, where $\Sigma$ is a self-intersecting switching
manifold. A double regularization of $X$ is a 2-parameter family of smooth
vector fields $X_{\e.\eta}$, $\e,\eta>0,$ satisfying that $X_{\e,\eta}$
converges pointwise to $X$ on $\R^n\setminus\Sigma$, when $\e,\eta\rightarrow
0$. We define the sliding region on the non regular part of $\Sigma$ as a limit
of invariant manifolds of $X_{\e.\eta}$. Since the double regularization
provides a slow--fast system, the GSP-theory (geometric singular perturbation
theory) is our main tool.
