\times L^2(\Omega)$ s.t.</p>
$$ (d^{-1}\sigma,\tau) + (div \tau, u)  = \langle \tau \cdot n, g_D \rangle_{\Gamma_D} \quad \forall \tau \in H_{0,\Gamma_N}(div,\Omega)$$$$ (div \sigma, v)                =  -(f,v) \quad \forall v \in L^2(\Omega) $$<p>where</p>
<p>$$H_{g,\Gamma}(div,\Omega) = \{\sigma \in H(div,\Omega); \sigma \cdot n = g  \text{ on } \Gamma \subset \partial\Omega \}.$$</p>
<p>The unknown $\sigma = d\nabla u$ is approximated using the incomplete P2 Raviart-Thomas element (RT1) and $u$ by piecew