Laplacian : laplacian

laplacian takes two arguments : an expression F of n real variables and a vector of these variable names.
laplacian returns the laplacian de F, that is the sum of all second partial derivatives, for example in dimension n = 3:

$$\nabla^{2}_{}$$(F) = $${\frac{{\partial^2 F}}{{\partial x^2}}}$$ + $${\frac{{\partial^2 F}}{{\partial y^2}}}$$ + $${\frac{{\partial^2 F}}{{\partial z^2}}}$$

Example
Find the laplacien of F(x, y, z) = 2x²y - xz³.
Input :

laplacian(2*x^2*y-x*z^3,[x,y,z])

Output :

4*y+-6*x*z