勾配
gradient
$ \mathrm{grad}
$ \nabla\coloneqq\left(\frac{\partial}{\partial x}, \frac{\partial}{\partial x}\right)=(\partial_x, \partial_y)
一般の$ n変数関数$ f(x_1, x_2, \cdots, x_n)に対しては
$ \nabla f =\left(\frac{\partial f}{\partial x_1}, \frac{\partial f}{\partial x_2},\cdots, \frac{\partial f}{\partial x_n}\right)
となる