Δf = div grad f

Note 1 to entry: In orthonormal Cartesian coordinates, the Laplacian of a scalar field quantity is:

${\displaystyle \Delta f=\frac{{\partial }^{2}f}{\partial x^2}+\frac{{\partial }^{2}f}{\partial y^2}+\frac{{\partial }^{2}f}{\partial z^2}}$.

Note 2 to entry: The Laplacian of the scalar field f is denoted Δf or ∇²f, where Δ is the Laplacian operator.