\frac{ \partial f }{ \partial \boldsymbol{x} } = \left( \begin{array}{ccccc} \frac{ \partial f }{ \partial {x}_{1} }, & \cdots & \frac{ \partial f }{ \partial {x}_{i} }, & \cdots & \frac{ \partial f }{ \partial {x}_{n} }\ \end{array} \right)^T \in \mathbb{ R }^n \begin{eqnarray*} \frac{ \partial }{ \partial \boldsymbol{x} } \boldsymbol{a}^T\boldsymbol{x} &=& \boldsymbol{a} \\ \frac{ \partial }{ \p