Recently, we tried to apply the GetFem++ to a problem governed by heterogeneous Helmholtz equation. We can find the system matrix by the generic assembly procedures. However, we don't known how to impose radiation boundary condition which involves high-order mixed derivatives (see
http://cedb.asce.org/cgi/WWWdisplay.cgi?138489 for details). We will be very glad to have your suggestions.
On the other hand, If we have a function on grid points, what is the easiest way to interpolate it and get its derivatives? Do we need to establish a mesh and then interpolate it?