Gradient of PDE solution
Row i from 1 to N of ux contains
Row i from 1 to N of uy contains
There is one column for each triangle in t in both ux and uy.
The geometry of the PDE problem is given by the mesh data p and t. For details on the mesh data representation, see initmesh.
For a description of the format for the solution vector u, see assempde.
The optional argument sdl restricts the computation to the subdomains in the list sdl.