Plotting derivatives of B-splines

Question

I am trying to solve an electrodynamics problem numerically by using B-splines a basis function. Please don't ask me why, but if you really wanna know, here is the assignment.

Using the following code, I have successfully plotted the B-spline basis functions in two dimensions, given the specific knot sequence and the respective x and y intervals.

knots = {0, 0, 0, 0, 0.2, 1, 1, 1, 1}
Plot3D[Evaluate[Table[BSplineBasis[{4, knots}, i, x], {i, 0, 3}]] Evaluate[
Table[BSplineBasis[{4, knots}, i, y], {i, 0, 3}]], {x, 0, 1}, {y, 0, 1}]

The corresponding plot looks like this:

Now I am trying to do the same for the second derivatives of the B-splines, using the following code:

knots = {0, 0, 0, 0, 0.2, 1, 1, 1, 1}
Plot3D[Evaluate[Table[D[BSplineBasis[{4, knots}, i, x], {x, 2}], {i, 0, 3}]] 
Evaluate[Table[D[BSplineBasis[{4, knots}, i, y], {y, 2}], {i, 0, 3}]],
{x, 0, 1}, {y, 0, 1}]

However, it does not work and produces an empty graph and lots of red error messages. I have tried several combinations of the D, Table and Evaluate operators, but nothing seems to work. What am I doing wrong?

Answer 1

knots = {0, 0, 0, 0, 0.2, 1, 1, 1, 1}; e1 = 
Evaluate[Table[
D[BSplineBasis[{4, knots}, i, x], {x, 2}], {i, 0, 3}]]; e2 = 
Evaluate[Table[
D[BSplineBasis[{4, knots}, i, y], {y, 2}], {i, 0, 3}]]; Plot3D[
e1*e2, {x, 0, 1}, {y, 0, 1}]

Plot3D[Evaluate[
  Table[D[BSplineBasis[{4, knots}, i, \[FormalX]], {\[FormalX], 2}] D[
      BSplineBasis[{4, knots}, i, \[FormalY]], {\[FormalY], 
       2}] /. {\[FormalX] -> x, \[FormalY] -> y}, {i, 0, 3}]], {x, 0, 
  1}, {y, 0, 1}]