 \hat{k}(\overline{\mathbf{x}}_j,\mathbf{x}_n),                  & \overline{\mathbf{x}}_j \in \overline{\mathbf{X}}, j = i;\\                 0, & \overline{\mathbf{x}}_j \in \overline{\mathbf{X}}, j \neq i.                 \end{array}             \right.\\     \frac{\partial{\hat{\mathbf{K}}}}{\partial