Hi Guys,

I would appreciate some pointers on numerical differentiation of interpolating functions in V10. What I really want to do is obtain Grad[f] where f is an interpolating function over R2. Using Grad did not work for me, so I tried ND, and it didn't either. I suspect it has to do with how and when it's evaluated. Is it possible to talk V10 into applying Grad, D, and/or ND to an interpolating function over Rn?

In[5]:= t = 
  Flatten[Table[{x, y, x^2 + y^2}, {x, -1, 1, .1}, {y, -1, 1, .1}], 1];

In[9]:= ft = Interpolation[t];

In[11]:= ND[ft[x, .5], x, .25]

Out[11]= ND[
 InterpolatingFunction[{{-1., 1.}, {-1., 1.}}, <>][x, 0.5], x, 0.25]