complement of a closed part D of its boundary. We prove that a function u $\in $ W 1, p
($\Omega $) vanishes on D in the sense of an interior trace if and only if it can be
approximated within W 1, p ($\Omega $) by smooth functions with support away from D. We
also review several other equivalent characterizations, so to draw a rather complete picture
of these Sobolev functions vanishing on a part of the boundary.