I know from definition that if some vector function $\mathbf{u}$ is given in three dimensional space, then curl is defined by this
$\operatorname{curl}\mathbf{u}=\nabla\times \mathbf{u}=\left|\begin{matrix}\mathbf{i} & \mathbf{j} & \mathbf{k}\\ D_x & D_y & D_z\\ u_x & u_y & u_z\end{matrix}\right|$
but unfortunately I forgot what represents subscript $D_x$. Is it the same as $u_x$? Because last one represents partial derivative and first one what is it?
Please help me.