Let \(U_{0}\) be the potential operator of a \(\mathrm{BM}^{d}\) in dimension \(d=1\) or \(d=2\). Show that every \(u \in \mathfrak{D}\left(U_{0}ight)\) such that \(u \geqslant 0\) is trivial, i.e. \(u=0\).