  Let $n\ge2$ and $\mathcal{L}=-\mathrm{div}(A\nabla\cdot)$ be an elliptic
operator on $\mathbb{R}^n$. Given an exterior Lipschitz domain $\Omega$, let
$\mathcal{L}_D$ and $\mathcal{L}_N$ be the elliptic operators $\mathcal{L}$ on
$\Omega$ subject to the Dirichlet and the Neumann boundary {conditions},
respectively. For the Neumann operator, we show that the reverse inequality
$\|\mathcal{L}_N^{1/2}f\|_{L^p(\Omega)} \le C\|\nabla f\|_{L^p(\Omega)}$ holds
true for any $p\in(1,\infty)$. For the Dirichlet operator, it was known that
the Riesz operator $\nabla \mathcal{L}_D^{-1/2}$ is not bounded for $p>2$ and
$p\ge n$, even if $\mathcal{L}=-\Delta$ being the Laplace operator. Suppose
that $A$ are CMO coefficients or VMO coefficients satisfying certain
perturbation property, and $\partial\Omega$ is $C^1$, we prove that for $p>2$
and $p\in [n,\infty)$, it holds $$
\inf_{\phi\in\mathcal{A}^p_0(\Omega)}\left\|\nabla
f-\nabla\phi\right\|_{L^p(\Omega)}\le C\left\|\mathcal{L}^{1/2}_D
f\right\|_{L^p(\Omega)} $$ for $f\in \dot{W}^{1,p}_0(\Omega)$. Here
$\mathcal{A}^p_0(\Omega)=\{f\in \dot{W}^{1,p}_0(\Omega):\,\mathcal{L}_Df=0\}$
is a non-trivial subspace generated by harmonic function in $\Omega$ with zero
boundary value.
