
\begin{aligned}
f(\mathbf{x}) &amp; = x_1 (a_{11} \mathbf{u}_1 + a_{21} \mathbf{u}_2) \\
              &amp; + x_2 (a_{12} \mathbf{u}_1 + a_{22} \mathbf{u}_2) \\
              &amp; + x_3 (a_{13} \mathbf{u}_1 + a_{23} \mathbf{u}_2)
\end{aligned}
