Laplacian : laplacian

### 2.50.2 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:

∇2(F)= |
∂2 F
∂ x2
| + |
∂2 F
∂ y2
| + |
∂2 F
∂ z2

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`